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All SV's have the 2.9 rear drive gears and I get 21 MPG when measuring with 100% highway driving at the pump (not the dash computer). All SL's and Platinums automatically have the 3.3 rear drive gears (2010-2015) and I have read many posts on here where everyone seems to get 16-17 on the highway - even worse.

Could this be because of the 2.9 rear drive gearing and smaller wheels?

I instinctively know that the 2.9 gears will get better MPG's if all other things are equal. But they are not, because ALL SL and Platinum wheels are increased to 20" versus the SV's 18". So shouldn't the larger tires pick up the slack? ...since we're getting 20" of distance instead of 18" with each shaft revolution?

The 2.9 gears turn 2.9 revolutions to travel 57" (18" * Pi = circumference) and the 3.3 gears take 3.3 revolutions to travel 63" (20" * Pi = circumference). With some quick arithmetic, we find that:

2.9 gears/18" wheels require 1,112 drive shaft revolutions to travel 1 mile;

3.3 gears/20" wheels require 1,006 drive shaft revolutions to travel 1 mile

...which are almost identical (considering how many revolutions there are)! MPG's should be about the same, and the EPA reports that it is!

One last thing: it's instinctive that increasing the size of the tire would give you better MPG on the highway at stable speeds. But when I researched this online, I found that the larger tire WOULD except for one thing: the gain is canceled out by something called Rolling Resistance. Since the larger tire has a larger footprint, the resistance and friction are increased too much to realize the gain from the larger diameter! The energy needed from an engine to overcome this added rolling resistance basically yields an overall LOSS of MPG's from upgrading just tire size (besides a little added weight)!

So I wonder if SV's do indeed get better MPG's than SL's and Plat's, is it because they have 20" tires instead of the 18" tires? If anyone has an SV, I'd love to know what your 100% highway MPG's are.

Thanks! ...any input is welcome.