Take the Vehicle Manufacturer’s Recommendation
We mentioned earlier that vehicle manufacturers often have a recommended tire size and type for each vehicle. This recommendation is based on the size, weight, load capacity, off-road capability and steering for your specific vehicle. Changing the tire size and type could impact the handling of your vehicle. We talked about how plus sizing a tire can change certain aspects of your driving experience.
Changing the tire size can also affect your speedometer reading. For many cars, the speedometer reading is based on one full revolution of the tire on your vehicle. If the tire size is changed, then the time it takes a tire to make one full revolution will increase [source: Yahoo Autos]. Since the speedometer rating is calibrated for only one length of rotation, a newer tire that is larger will inhibit the speedometer from reading the correct speed of the vehicle.
If you have an electronic automatic transmission, changing your tire size can also impact the timing of your shifts [source: Yahoo Autos]. This may impact your fuel mileage, uphill and downhill transmission changes as well as the general shift timin….click for full reading