mpg formula? - Page 2 - PeachParts Mercedes-Benz Forum

brewtoo · #16 05-21-2006, 10:46 AM

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.

whunter · #17 05-21-2006, 11:51 AM

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.

A264172 · #18 05-21-2006, 12:24 PM

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"

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.

t walgamuth · #19 05-21-2006, 12:32 PM

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

joselu43 · #20 05-21-2006, 12:48 PM

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

Roncallo · #21 05-21-2006, 02:58 PM

Quote:

Originally Posted by brewtoo

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?

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

Roncallo · #22 05-21-2006, 03:22 PM

Quote:

Originally Posted by brewtoo

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?

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

t walgamuth · #23 05-21-2006, 03:32 PM

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

DANSMB · #24 05-21-2006, 06:45 PM

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

rchase · #25 05-21-2006, 08:56 PM

Quote:

Originally Posted by t walgamuth

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

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.

rchase · #26 05-21-2006, 09:06 PM

Quote:

Originally Posted by brewtoo

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.

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...

t walgamuth · #27 05-21-2006, 10:10 PM

Quote:

Originally Posted by rchase

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.

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

Diesel Fan · #28 05-22-2006, 01:31 AM

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?

Randy · #29 05-22-2006, 10:20 AM

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.