Relationship between MPG increase and drag decrease?
 

Has anyone run across a formula or rule of thumb that correlates increases in MPG to drag decrease? Yes, there are a lot of other factors that combine to generate drag, but is there even a rough rule of thumb?

the mpg calculator on this site is the granddaddy of all.

aerohead has posted many a time about a rule of thumb for fe increase/drag decrease. But I don't remember it just now. :/

~2/3 for highway.

MPG % increase is half of aero % increase.

And it's nice to explain in more detail why this is....

I have done numerous coast down tests with the Insight and Crr and Cd are about equal at 35 mph or so.

This means if you change the aero of your car and make it 100% more slippery to the wind, it will gain you 50% more gas savings, because while the aero part improved 100%, the Crr part stayed the same. Then when you add the new Cd and the old Crr, you have a 50% net gain.

Jim.

Caveat
 

I just wanted to say that the 50% rule applies at 55 mph.
At 70 mph,a 10% drag reduction will net a 6% mpg improvement.
If you're driving in Wyoming or the Trans-Pecos region of Texas,at 80 mph,it will be even more.

Aero drag completely eclipses rolling resistance at higher speeds though. The reason is because the lower the load on your engine (generally, in the bottom 50-80% of torque output depending on forced induction or cams and stuff), the less efficiently it runs.

Thanks, I didn't know how the rest broke down because honestly I can't do a lot to change it. Now, aero, that's something to easily improve upon :)