Tire Size & Odometer Measurements

[Fotheringay-Phipps]

Apparently it is pretty accepted (& commonsensical) that changing tire sizes on the same car will throw the odometer off. My question is about cars that have two optional tire sizes. Do the manufacturors make two differently calibrated odometers that they install based on whether the particular vehicle is being equipped with larger or smaller tires? Or is the difference typically small enough that they don't bother with. (In the case of the car at hand, the difference is 2.2%).

[drdisque]

when they difference is as big as 2.2%, they usually use a different ECU, which they would have to do anyway if the car has a different drivetrain.

Remember, these days, the odometer and speedometer is just a dummy unit connected to the ECU, it has no "calibration" of its own.

[Fotheringay-Phipps]

I don't know that the versions have different drivetrains. The cars in question are the 2000 Prizm and Prizm LSi, two virtually identical cars, with some upgrades in the latter version. Based on the Tire Rack links in this forum, it would appear that the Prizm has an option of either tire size, while the Prizm LSi has the larger size as standard.

http://www.tirerack.com/tires/dg/SelectTireSize.jsp?autoMake=Chevrolet&autoModel=Pr izm&autoYear=2000&autoModClar=

http://www.tirerack.com/tires/dg/SelectTireSize.jsp?autoMake=Chevrolet&autoModel=Pr izm+Lsi&autoYear=2000&autoModClar=
What got me to wondering about it is that I have the LSi version and for whatever reason I took it into my head to measure the odometer against the highway mile markers. And best as I can estimate it over a 20 mile course, my odometer moves about 19.64 miles in 20 miles. This got me to wondering if my odometer had been set up for the smaller tire size and was thus incorrect for the larger size.

On the other hand, I would think the difference between a brand new tire and a worn out tire might be as big as, or bigger than, 2.2%, so I wondered if maybe the car makers are unconcerned about differences of that magnitude.