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  #1  
Old 05-19-2006, 07:00 PM
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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.

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  #2  
Old 05-19-2006, 10:14 PM
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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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  #3  
Old 05-20-2006, 12:03 AM
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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.
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  #4  
Old 05-20-2006, 12:15 AM
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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
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  #5  
Old 05-20-2006, 01:59 PM
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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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  #6  
Old 05-20-2006, 06:00 PM
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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
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  #7  
Old 05-20-2006, 07:54 PM
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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.
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  #8  
Old 05-20-2006, 10:53 PM
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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
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  #9  
Old 05-21-2006, 12:32 AM
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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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  #10  
Old 05-21-2006, 12:43 AM
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Go too slow and it will take you too long to get to your destination, thus requiring more fuel
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  #11  
Old 05-21-2006, 01:28 AM
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Quote:
Originally Posted by msethk
... 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.
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.
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  #12  
Old 05-21-2006, 01:51 AM
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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
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  #13  
Old 05-21-2006, 10:06 AM
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Quote:
Originally Posted by rchase
To answer some of the original poster's questions you can do some things to improve your fuel consumption.

-keep your car clean
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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  #14  
Old 05-21-2006, 10:13 AM
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Quote:
Originally Posted by Roncallo
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

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.
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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  #15  
Old 05-21-2006, 10:39 AM
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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


Last edited by joselu43; 05-21-2006 at 10:46 AM.
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