Differences between E70 & F15 35D that effect MPG

[Z06Luke]

I was noticing that the F15 35D is rated at 5 mpg higher on the highway than the E70 is. That's a considerable increase. I have tried to find out what changes were made that account for the MPG increase but I have been unable to find much information. The only changes that I am certain of that I can think that might have any contribution are as follows:

-Aerodynamics - We all know there were styling changes, but I do not believe the relatively minor design changes could possibly have a very significant of effect on economy.

-Transmission gearing - The F15 got the 8 speed auto, although the 8th gear ratio is very similar to the 6th gear ratio in the E70 (.69:1 compared to .67:1). This is nearly a zero change and would have minimal effect on MPG.

-Axle gearing - This is the only major difference that I can see. The E70 has a 3.64:1 final drive, where the F15 has a 3.15:1 final drive. Using a 255/55/18 tire (29" diameter), the E70 turns 1750RPM @ 60mph, where the F15 turns 1500RPM @ 60mph.

Can a change of 250RPM truly be the only significant change that provides a gain of 5 MPG?