mpg formula?
 

I have been wondering recently what the best speed is to drive my car. I drive a 1994 c280. with gas prices the way they are, there has to be a formula to determine (no matter what octane you run) what speed you will get the best gas mileage? any ideas? I've always heard somewhere around 50-60 mph but haven't seen anything scientific.

I think it has to do with air resistance mostly. Air resistance is the predominant form of resistance at higher speeds. I would guess a car with a good coefficient of drag <.33 for example maybe 50 mph is the best speed. A car with worse drag maybe 40 is the best speed. Assuming level ground of course. I get really good mileage at 80 mph down hills but high speeding tickets...

Mike

The 55mph speed limit and fuel economy has come mostly from politics. There are many factors that affect MPG such as elevation tempature and if the car is going up or down hill. There is no "one" speed that gets optimal MPG unless your driving on a flat road or on a dynamometer. The car's shape coefficient of drag and engine size would affect this as well.

I do a bit of flying in private Aircraft and their GPH figures are not based on formulas. When the plane is in its test phase pilots from the test firm will take the plane out and do several hundred flight tests and the data from these tests are logged and put into a graph which is a rough at best guide to the Aircrafts performance. In aircraft engine performance you get a much larger range of conditions that the aircraft is subjected to. When you go from sea level to 10,000 to 20,000 feet within a matter of minutes your engine performance and fuel economy changes fast. Pilots have to keep up with these variations in real time to make sure that they have enough fuel to complete their intended route. Some smaller private planes based at airports in higher altitudes can be grounded by variations of barametric pressure. When the pressure changes these planes don't have enough power to safely take off on a certain length of runway.

Personally I wish car makers would realease the inforamtion like aircraft makers do. However in our politically charged world with lawsuits being filed for any reason its too much of a liability. If your car has an optimal MPG at 75mph and you put that in the manual and your customer gets in a deadly accident your going to get sued. Im sure Mercedes has done tests that could be converted into a graph to show you your optimal speed vs mpg but that information is locked in a filing cabinent somewhere and will never be released due to the sue factor and the lack of demand for the data.

To answer some of the original poster's questions you can do some things to improve your fuel consumption.

-keep your car clean
-tire pressure
-use cruise control
-go with the flow with traffic
-keep your car maintained and operating at optimal conditions
-avoid sudden bursts of power

the slower the better. drive 40 mph and you will get excellent mileage. so drive as slowly as you feel safe in traffic.

there is no car that i have ever heard of that gets better mileage at 75 than 60 because of some mythical "sweet spot".

tom w

but not too slow, you will very poor mileage below 20 mph, I am sure. The optimum speed will change from car to car. Around 40 mph is probably a good number for most cars. In city trafic it is better to coast to a red light than to stay on the accelerator and then brake. If you can time the lights so much the better. I agree that there are no magical sweet spots at high speed. An no formulas either.

JL

The slower you go (within reason) the better your mileage will be.

I just looked back at the manual I have for a 380 SEL which includes a fuel consumption graph. The line starts on a relatively flat path at around 40 km/hr with fuel consumption at 8.5 L/100km This indicates that the best MPG for this car would be approx. 30 MPG @ 25 MPH.
As the speed is increased, so is the fuel consumption, and it increases exponentially.
The graph shows fuel mileage to be 24 MPG @ 55 MPH, and 9.4 MPG @ 125 MPH.

There is no "flat spot" in the graph where fuel consumption is the same or lower with increased speed.

If you were to design a car for optimal gas milage, slower is the way to go. There are two components of resistance that have to be overcome by power. The rolling resitance and the areodynamic drag. Power required to overcome rolling resistance is roughly proportional the speed of the car. For example if it takes 5 HP to go 50 mph it will take about 10 HP to go 100. Power required for areodynamic drag is proportional to the qube of speed. If it takes 5 HP to go 50 mph then it would take 125 to go 100 mph. Therfore a car with the above performance would require the sum of these powers

5HP rolling power + 5HP areodynamic power = 10 HP @ 50 mph
10HP rolling power + 125HP areodynamic power = 135 HP @ 100 mph.

10 HP is approximatly the required amount of power to drive the average car @ 50 mph on a level surface in no wind at sea level with standard atmosphere. So the above example is somewhat typical.

So from the cars point of view the power required is absolute minumum the slower you go. Now the engine and transmission put into todays cars are not very efficient at low speeds. This is because the engine is way off its power curve and the transmission's torque converter is slipping. Genreally a standard engine will have its best specific fuel consumtion SFC at peak torque RPM, at wide open throttle WOT, which is typically in the 70 mph range for most cars. The best SFC will occure at lower RPM's for off WOT conditions. The transmission will not lock up its torque converter under about 30 mph. so while the car is getting less efficient rapidly with speed, the drive train is getting more efficient.

I would estimate the optimum fuel economy for most US design cars occures in the 30 to 50 mph range for cars with automatic transmissions and 20 to 50 for cars with standards transmissions.

I would also say that German cars are more likely to get better milage at slightly higher speeds due to the design focus including driving on the Auto Bahn.

Stuff like variable valve timing and 7 speed transmisions allow the drive train to perform better over a wider range of conditions and therefore will result better gas milage at lower speeds.

John Roncallo

the smart driver is the best tool that aids in fuel economy.


and i can sum up how in two words; maintain momentum.

this means timing lights, coasting to a stop, letting the car roll faster on dowhills, accelerating downhill if there is an up hill after it, letting it slow down on long up hills, getting to cruising speed as quickly as possible, staying in top gear as long as possible and so on.

conversely, there are other things you can do, like increasing your air pressure, getting a tuneup, keeping your windows and sunroof closed, using your AC minimally, removing all the unecessary detritus from your trunk, combining trips, avoiding traffic (i used to go to the gym near work, then go to work to avoid being in traffic if i left later!) and so on.

Go too slow and it will take you too long to get to your destination, thus requiring more fuel ;)

My 2000 E430 has trip computer, I tried several speeds using cruise control for one mile each on level surface, the lowest speed was 45 mph and the highest speed was 75 mph, with inscrement of 5 mph. The best gas mileage was 33.xx mpg at 45 mph, 32.xx mpg at 50 mph, 30.xx mpg at 55 mph, 29.xx mpg at 60 mph, 27.xx mpg at 65 mph, 24.xx mpg at 75 mph. I don't remember the gas mileage at 70 mph. I did not try 40 mph. The EPA of 20000 E430 was 18 mpg city and 24 mpg highway.

All gas mileages above was after the car totally warmed up after 20 minutes drive. The worst gas mileage I found is the first 10 minutes when the engine was cold.

Others were correct to point out that: don't carry extra unnecessary loads, increase tire pressure 2-6 pounds above recommendation, use cruise control whenever possible, cruise to stop light ... would improve gas mileage.

We wouldn't need engines if we could figure a way to create a continuous vacuum just ahead of our cars ..... the air pressure behind would simply push us along .....

Sorry .... to disrupt the thread.

Haasman

Since someone earlier is this thread was talking about airplanes, I'll add this...

If an airplane has frost on its wings, it will not fly because of the disruption in airflow over the wing.

However, a clean (or even freshly-waxed airplane is no faster than a dirty one. An airplane with dead bugs all over the leading edges is no slower than a clean one.

So I'm not sure keeping a car clean makes any real difference in MPG.

Very good. Most people don't realize this. However, I was taught that when the speed doubles, the force of the wind quadruples (X4, not X5).

Also, does your car require WOT at 70MPH? If so, wouldn't 70MPH be your top speed?

A dirty airplane will require more power to fly at the same speed as a clean one. In airplanes flight speed (or Mach number) is pre set (depending on a lot of variables like weight, altitude etc). The pilot or autopilot will apply the required power to maintain it. A dirty airplane is less efficient that a dirty one and will use more fuel. I agree that the effect may be negligible for a car, since the car is aerodynamically a lot less efficient and the speed is much lower, drag increase may be unnoticeable.

On the power, John is right. As the velocity doubles, the force quadruples as brewtoo says, but the power required is eight times larger (X8)

Jl

I attended a seminar by Roy LoPresti. He was a major factor in the redesign (for speed and efficiency) of the mooney aircraft (J model) and has been involved in numerous speed mods for different.

He stated that their research had shown that the advantage of a clean airplane over a dirty one was negligible. Same with bugs on the leading edges. Yes, it surprised me, too.

In my personal experience, I have never been able to notice any difference in fuel consumption (or performance) whether the plane was dirty or clean.

YMMV, of course. :D

Rofl
 

Funny, this topic pops up every time fuel cost goes up. ;)

The factors are a huge list, (that can NOT be calculated except by massive testing of each specific single vehicle.

This is the shortest possible list of factors, a more complete list could be 500-2800 factors.

#1. Body style = air resistance.
#2. Vehicle weight.
#3. Vehicle Single driver profile only = minimum 250 hours of 180 channel normal road drive data = valid for that driver only.
#4. Fuel quality.
#5. Engine size + programming strategy.
#6. Transmission gear ratio + programming strategy..
#7. Differential gear ratio + programming strategy..
#8. Tire brand + condition + road surface condition.
#9. Tire pressure.
#10. Wheel bearing condition.
#11. Season/current climate conditions.
#12. Route = elevation or hills/mountains.
#13. Accessory use/load.

A good mechanic can improve base MPG.
The sad fact is that most drivers are beyond training = they feel the need to go zero to 130 MPH in seven seconds.



Have fun.

I remember reading a forum member's statement that "the slowest speed in the highest gear = the best fuel economy" and with some variation based on overall driving conditions "the faster you get to that speed the better" :cool: I believe this means that you want to spend the greatest amount of driving time at the most efficient speed ad if by going fast to get to this speed you will be forced off of it by road conditions then fast is not good but if you can get there and stay there then fast is good.

i agree except with the part about getting there as fast as possible.

for best mileage it is best to accelerate gradually and get to the next higher gear as quickly as you can wo lugging the engine.

in my dodge with cummins, if toodling around town i will shift at 2000 rpm and skip 1,3 and five gears.

if not pushed it will pull comfortably from 1000 rpm.

now a benz has a higher rpm range but the general idea is the same.

accelerate gently getting to the higher gears asap and coast at a stop sign and let the motor do as much slowing down as possible.

tom w

Some of the new "smart" auto transmission will upshift sooner when the force on the accelerator is light or you briefly lift, and hold gear if not. If you have one of those you can improve your mileage significantly by using this feature.


Jl

This is true for force not power. When speed doubles force quaduples (or is actually squared 2^2 = 4),. Aerodynamic force, or more acuratly drag, is a function of speed squared. But power is drag times speed. Therefore power is a function of speed cubed.

Its somewhat scary isint it. The good news is that drag is so small at low speeds that it is off the map. But above about 40 mph its all drag and it comes on quick.

John Roncallo

I forgot the WOT portion of this question.

The data reported for engines is typically reported at WOT but does not have to be. On an engine dyno the throttle postion is set usually to WOT and the engine external load is varried. Different loads will allow the engine to spin at different RPMs for the given throttle setting. The data for the engine is than maped at each load level where RPM, Torque and fuel cunsumption are measured. From the torque and RPM measurements HP is calculated. From the calculated HP and fuel cunsumption SFC is calculated.

Most cars do not require WOT to go 70 mph, therefore the best SFC map for an installed engine would have to come from a part throttle dyno run. In this part throttle dyno run the RPM where SFC is minimum would most likely be lower than the RPM for best SFC at WOT.

John Roncallo

about 20 years ago i had a 1980 datsun 210 wagon with a 1300 engine and a five speed. i bought new tires for it and thought it would be smart to go from 155 to 165 tires. you know a little better economy from a slightly higher effective gear ratio.

well, that little sucker was already pretty optomised with the stock size tires 'cause after the change it would only do about 65 flat out in fifth. it would go considerably more in fourth and if i were to acclelerate to 75 in fourth and get behind even a 240d (which i did once) the draft would allow travel at 75 in fifth easily.

and it would deliver consistant mileage over 30.

but after a trip once when i was supposed to meet someone at a particular place at a particular time and had a headwind and barely could exceed 45, i decided that such a price was too high to pay for max economy.

so there is the rare example of a car that would be wot at seventy (or less).

i never would have dreamed you could benefit from drafting an ordinary car. but the datsun was pretty bricklike.

tom w

My wifes SL 320 1997 has a mpg gauge that is really a vac gauge but it reads the highest mpg at approx 50 mph. My 2002 Dwnali pickup has the instant and average mpg readings and same thing, 50 mph is best-17 mpg, 60mph shows 16 mpg and 70 mph shows 14mpg!

Dan

Hmmm,

With your logic a car parked in its garrage gets the best MPG (try dividing 0 miles by how ever many gallons it takes sometime for fun). What science do you base your claim on? You could be driving 5 miles an hour up a steep incline with a trailer attached to the back and suck fuel like it was going out of style. Speed really has nothing to do with fuel consumption.

The formula is miles per gallon. Essentially its how many miles you can drive on a gallon of gas. The engine consumes more fuel the higher the load and engine RPM. There are many factors that can affect engine performance other than the speed the vehicle is moving.

Here's an experiement everyone with a gas MB can do to show the speed vs fuel economy. Set your cruise control on an open road to a reasonable speed. Anything you want slow or fast. When your car goes up a hill the MPG indicator will drop and when you go down the hill it will go up and probably peg the indidator. What you just saw was the load on the engine change. When the car is going up the hill the engine has a heavy car its trying to move up the hill and has a load on it. When the car is going down the hill there is no load and the weight of the car actually helps to pull it down the hill.

So there is no "Optimal" speed because the road conditions are unpredictable. Unless you have fuel consumption data on thousands of test drives done in labs and a small supercomputer in your back seat its impossible to determine "optimal speed". Unless your just happy with a "guess".

Nothing annoys me more than some Prius driver driving in the outer lanes doing under the speed limit trying to eek out an additional .0001 MPG. The thing is MPG is an averaged figure thats based on an entire tank of gas. That one time you nail it trying to pass someone you have just affected the average even for that small amount of time you had your foot in it. Driving slow only slows people down and consumes more fuel. Nothing is more of a waste of fuel than cars that are stopping and going in traffic. People who drive slow cause accidents and slow down the flow of traffic. By driving like a moron to save yourself a penny or two your causing a lot of problems for many other people.

Hmmm I wonder if he would say the same thing for little pellets of ice on the leading edges of wings..... It really sucks flying around with the stall horns screaming at you everytime you try to climb more than 50FPM... <evil grin>

I agree the effects are negligable on a private plane like a Mooney or even a car but I wonder how it would affect things if that Mooney could do 500KTS. Oh wait the wings would be ripped off at 500KTS. Air while it does not seem like much is quite a force. Air resistance is an actual force that vehicles deal with.

On cars its the reason why so many of them are wind tunnel tested and its why we don't drive square cars. Ok whell I do drive a square car but thats beside the point... :)

whatever.

personally i drive about 10 mph over the speed limit about all the time. i was not suggesting that anyone drive 40 mph on the highway. i was answering a question about what speed to drive to get the best mileage.

as they say in the army.... explain however you want and 10% will never get it.

can you tell i don't like being called a moron?

tom w

Neanderthal sums it up nicely. Also two big things. DON'T use the cruise except to relieve fatigue. Except on lazer flat roads, it wastes fuel.. Also, are you really prepared to save a few dollars by driving 50 MPH? If so, you need to reevaluate what your time was worth. If you were an executive in my employ and you drove the company Mercedes 50 mph to save a few bucks I would replace you. If you save two gallons an hour at $3.00 per gallon then you must consider your time to be worth $6.00 an hour. Can you buy a MB on that wage today?

What is really of higher value?
 

fuel or time?
Time is the higher value.
If you are cheap, drive smoothly at the lowest speed possible in high gear. Imagine there is an egg between your foot and the accelerator pedal, and try not to crack it.
And, above all, Don't Panic.

I thought the discussion was about about a formula for fuel comsumption ...

Jl

Huh?

:D

On level ground you should see your best fuel economy when you are traveling the slowest speed in top gear you can maintain without excessively lugging the engine. This results in the least wind drag and most distance covered per engine revolution.

If you really want good gas mileage, although NOT a good idea, drafting can make a big difference. But wreck the car once and there goes the gas savings.

it was a response to rchase who did a long thing ranting about slow drivers and ended up (it seemed to me) calling me a moron, indirectly.

my point being that he was point a 'slow' finger at the wrong person (me who has a proper germanic lead foot).

tom w

It was. Until the old wives tales started coming in.

Hmmm. You don't drive a Prius do you? While my critical comments on your "science" was for you the comment about the slow drivers was a broad statement meant for everyone who clogs the highway being cheapskates. For all of "those" people. If gas is so expensive then STAY HOME! :)

Sorry for the delay. Here is the formula:

( wattage of the stereo / 5 ) + 20 = optimum speed in MPH

Mike

if this is an apology, thank you.

i am sorry but i find your way of saying things difficult to understand.

tom w

Down shift cable
 

Friends,
Most older MB's have a downshift/throttle cable to signal the trans. when to shift. If you lenghten the effective length of that cable the trans. stays in the higher gears longer and upshifts sooner at the expense of power.
dpk

Hey Mike, are you sure about the formula? I don't think it is dimensionally correct. You need a fudge factor man!
Works for my car though ...

JL

this is one of the weirdest discussions I've seen in a long time. All this pontificating and formulizing may yield you what?, 5 miles per tank? Do you really want to change your driving habits that much and stress over an extra mpg? Not to mention that your car will be covered in spit if you're the guy driving down the freeway going 40mph holding up traffic just because you want to optimize your fuel economy.

OK, with that said, when I want to optimize my mileage, I don't look too much at the actual speed I'm travelling, I try to minimize my RPM's. For example, when starting from a stop, gather speed gradually rather than punching it. I try to keep my RPM's below 2,800 at all times and it really saves a lot of gas.

But come on, who's really going to drive a $60,000 car down the highway at 55 MPH?

I do try to use common english. It was not really an apology as it was not my intention to insult you in the first place Tom. But I am not really happy that you felt slighted or insulted by my post so please do consider it an apology if that is the case. It is somtimes difficult to challenge someone's ideas and concepts without them feeling like its personal.

I have read many of your posts in the past and you do have insightful and informative things to add to the discussions here.

Thats part of the problem I think. Some of the cars here that are being discussed are $2000 cars and fuel cost is much bigger an issue since people who drive those types of cars generally have tighter budgets.

If your really looking at fuel economy a Mercedes is probably not the car you should be driving anyway as even the diesels were not designed with economy as their primary trait. A lot of the VW diesels and Japanese cars get much better fuel economy and have a much lower operating cost.

The other thing to consider to save fuel costs is to just stay home. Americans have for years not thought about the cost of fuel because its so cheap. Now that fuel prices are high we want our cake and eat it too with the same amount of recreational driving that we had in the past for the same price. We try to maximize our fuel economy so we can still do everything that we did in the past. Perhaps instead of driving around on the weekends visiting friends and going to dinner you could have dinner at home with friends? Instead of runnining to the grocery store for fresh groceries every day you might want to buy in bulk. Everything we do in our lifestyles no matter how innocent sounding has a cost one way or another. Living in the suburbs might save you some money on a house but does cost you in transportation expenses. Most people in the burbs can't exactly walk to the corner market to buy groceries every day.

P.S. To those who might get insulted by the $2000 car comment I have to point out that there are many millonares driving around in $2000 cars. The president of Ikea and one of the richest men in the world drives around in a 1993 Volvo 240 wagon just like mine. Just because your cheap does not mean your poor! Although I don't consider being cheap a good personal quality myself.

P.P.S Something else to look at that affects your fuel prices. When you buy a product at the big box store take a moment and turn it over and look where its made. hmmm China eh? With a huge portion of our imports coming from China the chinese are becoming quite wealthy and are starting to buy cars. Those cars need fuel. So the great deal your getting on the plastic crap your about to buy is not such a great deal anymore when you can barely afford to drive anymore because of fuel costs.

cheap is buying an expensive meal and tipping the server 5 or 10%.

driving a car that gets good fuel mileage and buying it used is thrifty.

i pride myself on being sensible with my money, but i dont want to ever be cheap.

tom w

Let's not forget that mileage is made or broken not on the highway going a steady 55, or 65, or 85... It takes a beating on the acceleration, idling, stop and go, etc that it takes to get to the highway and in stop and go traffic. If you're heavy on the gas and brake, your mpg is going to suffer. So take it easy out there, pick a speed, stay with it and forget about weaving in and out of traffic all over the freeway just to get to point B a minute quicker. Keep your RPMs to a minimum and enjoy the ride

personally, i like to keep my pontificating concise.

tom w

i think I have gotten some ideas from this thread. I dont want to sound real liberal (tree hugger) i started this thread in an effort to decide how fast i set my cruise control when i drive across town here in tucson to get the best gas mileage. All drivers should do the best we can to conserve fuel no matter what our financial situation (nobody should have to "afford $3.xx plus fuel with oil companies making record profits.) Less demand should mean lower prices; especially until americans are smart enough to wasm up to the idea of newer diesels. I am really wainting for MB to start offering the c class as a diesel here in the states (what s their problem? would they rather offer a jeep liberty with a diesel with poor economy?). I remember reading about mid-1990's c class diesels in europe getting mid 40's mpg. BMW offers a 1,3 and 5 series bmw in a diesel in mexico europe and australia. when will they be here?

soon i hope.

nothing wrong with hugging trees. they provide us with oxygen. shade. and look pretty nice too.

tom w

Hmmm... Im not sure your going to see the diesel in a C class anytime soon..... Its the whole brand recognition thing in the American market...... Mercedes is being very cautious with the image of the three pointed star now that they own Chrysler and have brought back Maybach.... They don't want to do damage to a brand and reputation that they have taken decades to build...... If you notice the C coupe is completely gone and probably won't be coming back again..... A C diesel would be a low grade car that they would not be interested in selling to the US market...... The lower end cars for the American market will all have Chrysler logos on them from now on...... I would expect an S class diesel before a C class diesel... I think for now the E will be our only choice for years to come....

I was poking around a Chrysler dealer today because a friend was turning over his convertible for some service and noticed some interesting things..... Most every car I saw had "parts from Germany" for their transmission on the window sticker..... When I went into the service department with him I saw the A, B and C service setup like Mercedes does.....

Fuel prices are at the levels they are now because of choices that we have made in the past. While I don't see the oil industry being sympathetic to our cause I don't think they are up to anything really sinister here.... China needs more oil to produce the cheap goods we buy in record quantities and to power the cars being bought by the newly created Chinese middle class..... Because we are so greedy and want the cheapest price "no matter what" we end up paying for it in other areas.... High energy costs is the way the costs are displaced for buying cheap plastic crap at a big box store..... Ironicly one day we will get to the point that its more expensive to drive the cheap plastic crap home than it is to buy it....... Rather than do something useless like worry about what speed your cruise control is set to to try and save a few pennies on oil why not do something useful.... When your at the big box store next time take a look at the label and see where the product your buying is made...... Every penny you put into the Chinese economy is another penny for them to drive up your energy costs.....

I make an effort to avoid Chinese made goods at all costs. Its kind of fun too.... Lately I have been building up my Italian Made wardrobe.... While more expensive there's no comparison between a cheap production sweatshop made item of clothing and a handmade Italian tailored item.....

does mercedes realize the potential? vw cannot sell enough tdi's. for a majority of mercedes owners (outside sl, 450sel, and 6.9r's) i believe mercedes reputation in the u.s. was built on the reliability of the bulletproof diesel! New car owners making a statement are the reason so many dodge durangos are being traded in and demand is softening for big suv's. listen to the market MB!

Robert is onto something when he says it is a brand thing and that Mercedes is taking great care in protecting their marque reputation. Let us face it, as clearly seen in this thread, a great number of Mercedes owners are not concerned with fuel mileage and would not buy a diesel for that reason though they might buy them as part as an overall package. A lot of Mercedes buyers in the US buy a Mercedes because it is a Mercedes, period. They want the prestige they perceive goes with the brand. I do not believe this is true in Europe, there a Benz is just another luxury car and the Europeans are not prepared to pay a high premium for Mercedes cars as americans do, the market there is now dominated by diesels (--> fuel economy). So, I agree with Robert that the decision is one of marketing and not of mpg.

Jl