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.

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1994 C280
2009 VW Tiguan
1993 Toyota X-tra Cab SE-5


1973 220D ... Gone, but not forgotten
1991 Alfa Romeo 164L Gone, wife MADE me forget it

2006 Hyundai Tucson... just straight out FORGOTTEN!

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

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

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

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.

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.

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2012 E350
2006 Callaway SC560

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.

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'O=00=O'
bmw 2002.
long live the legend

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

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1985 CA 300D Turbo , 213K mi

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?

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2012 E350
2006 Callaway SC560

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.

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2012 E350
2006 Callaway SC560

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...

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...