What speed are the MPG tests done? The T rear spoiler is at a higher angle when deployed = more drag and lower MPG. The T has the S gearing as well with a mech LSD (more robust than eLSD adds small amount of drivetrain drag). All of those combined can easily reduce MPG but especially the spoiler angle.

 

Could the difference in mpg due to lawsuite on false mpg data? Maybe one set of mpg is accurate and the other is data that was inaccurate and the basis of the lawsuite.

 

The base model car comes with 19 in wheels and tires and the T comes with 20 in wheels and tires therefore affecting the rear final drive ratio. So for example the 20 in wheel and tire combo is going to have a higher numbered rear gear ratio to offset the taller tire/ rim setup and the 19 in setup will have the lowered rear gear ratio.

 

Three things

Different gear ratios on T
More aggressive aero on T with more drag
Base model without optional wheels came with more narrow and fuel efficient 295/35/19 and 235/40/19 tires verses T on wider 305/30/20 and 245/35/20

 

^^^ Wheels and Aero (spoiler height) + final drive.

 

Oh yeah, the tire width will have significant impact. I think stickier rubber too. On a previous car, I went from medium sticky 215 wide front tires (245 rears) to very sticky 245 wide front tires and rear tires. Highway mileage dropped from about 29mpg to 25mpg.

 

Silly question but is it possible to swap in the final gear from a T into a base?