Help please, fuel/gas economy at different speeds- Settle a bet!

[uppityperson]

Does anyone know of hard data that shows fuel economy at different speeds? I know that it will be different for each car, but I am trying to figure out if 55mph is considered the standard due to fuel economy, or if this was chosen for some other reason (like safety).

I have thought that 55mph was supposed to be the best speed (vs 65 or 75) because of fuel economy, but have a vague recollection of it being this for safety, and that there is not much mile/gal difference if you go faster. Someone else says they thought 40 mpg was best for fuel economy.

I searched, but keep coming up with the same information with no data to back it up, meaning it is meaningless except for legend's sake.

Thanks for helping settle the bet. (the bet is small, but still, for the "Told you so factor", I'd like to find out. And just because also.)

[Ravenseye]

1. Simple. It varies.

http://www.viragotech.com/fixit/FuelEconomyEngineEfficiencyPower.html

Sorry for the kind of inconsise answer but it differs from car to car. Depending on the weight of the car, the strength of the engine, it's aerodynamic drag, etc the best most fuel efficient speed varies depending on the car.

Google "Fuel Efficiency MPH" and you'll find a number more links to expound on it.

[HamstersFromHell]

2. The best *engine* fuel efficiency...

will occur at the engine's torque peak (in rpm). Gearing of the vehicle and wind resistance will also play into this.

If a car is relatively aerodynamic and the gearing is low enough, the most efficient vehicle speed could very well be 70mph or even higher. If it's a boxy vehicle and had to move a lot of air, this may not be true.

The reason you can't find a definite source for an answer is that there really is no definite answer. Each vehicle's aero drag, engine size, torque curve, transmission and rear end gearing and tire size (as well as rolling friction) has to be taken into account.

[Ragnar]

3. It's better than higher speeds

It was a political compromise during the oil shortage of the 70's. Fuel economy is improved by driving at lower speeds and more consistent. It is stoppping and starting which affects fuel economy. I think it likely that the ideal speed for fuel economy would be idling in top gear. In most modern cars, that's about 20-30mph, depending on the gearing.

[Racenut20]

4. Back in the 70's when it was used as the speed limit

It was said that 55 was the Mean economic speed, which of course would relate to vehicles of that era and not necessarily today. Those promoting 55 today have probably not checked the stats...or maybe they have and I have not read of it.

[sweetheart]

5. Consider the boundary conditions

at zero MPH, there are no casualties and 100% fuel efficiency
at 100,000 MPH, there are always casualties and enormous fuel is burned

The slower you go, the more efficient and the fewer deaths.

[unhappycamper]

6. I drive a 1998 Mustang V-6, and find I get around 30MPG

with the windows up, air conditioning on at 70MPH. Windows down at highway speed drops my MPG by at least 15%. It's all about aerodynamics in later cars.

[noamnety]

7. There's a graph here

http://www.fueleconomy.gov/feg/driveHabits.shtml

If the gov site is an acceptable source to the other person, you should win your bet easily.

[Ready4Change]

8. Lowest speed that allows the engine to run without harm in the highest gea

For example, 5th gear at around 1500-2000rpm.

For a lot of cars this will put you around 40mph. At that speed you are getting maximum wheel rotations for minimum engine rotations, while minimizing energy loss due to aerodynamic drag.

Just MHO.