Different tire sizes won't affect the trip computer, per se, as the computer calculates distance based on the number of axle/wheel revolutions, not the actual distance traveled. The upshot is:
...Auto manufacturers program most cars (speedometers/odometers/trip computers) to read "optimistic" by 2-3%. This means if your speedo reads 50 mph you're probably doing an actual 48.5-49.0 mph - - to keep you on the safe side of the law, etc.Opps, just realize I tyupe
...When your odometer reads 50,000 miles, you've really only gone about 48,500-49,000 miles.
...Thus, your MPG calculations are generally optimistic by 2-3% because you really haven't driven quite as far as you thought.
I lucked out with my aftermarket wheels/tires (225/40-18) because my speedometer is close to dead on, as confirmed by carefully observing my speedo when driving by several different roadside radar displays. So, even though my tire size results in accurate speedometer readings, the trip computer's average MPG computations are still off. I agree, it would be nice to fine-tune the trip computer's average MPG computations, but I won't hold my breath for that. (yawn)