[briank]

I read a message somewhere, where the poster concluded that the mpg while driving at (1) 60mph with a 10mph tail wind will give the same mpg as driving at (2) 40 mph with a 10mph head wind. His conclusion was probably based on the fact that in both cases the wind was flowing past the car at 50 mph as both scenarios cause the same effective air drag on the car. (To use aircraft analogies, air speed is 50 mph in both cases, the ground speed differs)

Now I would say the car in scenario (1) above is probably getting almost 50% higher mpg than (2) above because it's covering 1.5 times the distance with the same effective air drag. This improvement in mpg is far and above what on might expect by driving at 60 instead of 40mph with the wind scenarios above. Also for a more typical American car (automatic with larger engine), the car in (1) will be more likely in its engine sweet spot with regard to BSFC.

Personally, I definitely see the improvement in mpg with my 2008 2.4L Scion xB when there is a 10 mph tail wind, where I can average 45 mpg at 60 mph. Get this it goes down to 40 mpg when I reduce the speed to 55 mph, so going faster up to a point when you have a good tail wind is good for the pocket book. Going against the 10 mph wind going the other way I get 20 mpg at 60 mph, a reduction beyond what one might expect.

So I have yet to determine what is the best speed with a 10 mph head wind or any other head wind speed for that matter. The same for a 10 mph (or other speed) tail wind, what is the best ground speed.

Any answers to this would be appreciated.