Differences between E70 & F15 35D that effect MPG
 

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?

there is also an eco pro mode that can be selected on the F15, it limits acceleration

The first gen e70 had the 6-speed ZF, the facelift e70 already got the 8-speed ZF from about 2010, including the revised rear axle ratio (3.15 to 1).

The F15 got electric steering which , according to ZF and BMW was supposed to have fuel economy benefit.
Also the F15 has a completely different ECU engine map, compared to the e70.

F15 has the N57T engine - completely different and improved design. - E70 has the older M57.

You are describing gasser, not a diesel. A US-spec E70 35d's were made from 2009 to 2013 and cover pre-LCI and LCI bodies. All model years had the same 6-speed ZF tranny.


F15 has one less turbo and bit less power, at least on paper, vs E70. With tranny, electric steering, improved aerodynamics, you may get better mpg but really doubt that by whole 5 mpg.

Yes you are right the US 35d e70 were stuck with the 6 speed, I was thinking of the Euro X5 diesels, like the e70 40d which got the 8 speed