1990 740 turbo (still looking for EGR temp sensor)

I've never been able to get more than 20.5 mpg down here in the flatlands (central valley california) and it just occurred to me the other day that perhaps I AM getting more than that.
If the "mile" my car travels covers more ground than the "mile" another 740T covers, then we could travel the same distance, consume the same gas, and have different reported miles traveled. The odometer gets its data based on tire rotation, right? So if my tires are rotating fewer times than a similar 740, I would have reportedly traveled less miles and thereby gotten less mileage.

I was ready to spend part of my afternoon looking into this, but I need some imformation.

I think it was 195/65/14 but I'm not sure.

Anyway, I would think that the actual rotational surface of the cars would be kept the same, and the width of contact surface would be changed most.
I'm only speculating, does anyone know?


Thanks in advance!
-Will

PS I consistently got 22 mpg in the mountains between 2500 ad 5000 feet last summer and stayed in 3rd gear (AT) for almost the entire time for the engine braking and to avoid repeated downshifting.
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1990 740 Turbo, on its way to stock specs, maybe beyond