You'll only experience the type of problem described in post #1 if you have tires of 2 or more different sizes at the same time on your car, and if the size difference between them it outside the computer's happy zone.
I bet the computer itself wouldn't care if you put tractor wheels on all 4 corners, as long as they're all the same size.
Might rub, though.
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View my fuel log 2014 Mirage ES 1.2 manual: 62.4 mpg (US) ... 26.5 km/L ... 3.8 L/100 km ... 74.9 mpg (Imp)