Wheel and tire weight matter a bit, tire width matters a lot. Higher inflation helps reduce rolling resistance. Diameter shouldn't matter unless it is causing your speedometer/odometer to read low (deflating MPG).

Look at the rear tire size on the i8 - there is a hint in that for getting higher mileage.

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Mike

2005 e53 X5 4.8is 169k miles
2018 f90 M5
2018 Volvo XC60