On cars and trucks that have antilock brakes (ABS) equipped cars and trucks, most vehicle manufacturers say replacement tires should be the same size as the originals. This is necessary because the diameter of the tire affects how the wheel speed sensors read, which in turn affect the operation of the ABS (and traction control) system. Changing to a larger or smaller diameter tire, or installing different sized tires front and rear can upset the operation of the ABS system. This may create braking problems and/or cause the ABS warning light to come on (which means the system is deactivated). But I like the other people who have posted have changed wheel &/or tire size with no ill effects on different makes & modles, except for the speedometter error, mine right now is 5MPH faster then what I'm really doing. So I think what willg1966 posted " seriously no one knows?" is probley accrete.