Everything tends to be relative and a small amount off (one or two sizes) will normally not cause much damage. It will usually result in an incorrect speedometer reading and may effect transmission and ABS operation. Different size tires from side to side can cause problems. The shorter tire will have to rotate faster than the taller tire. This causes the differential to spider gears to spin and can cause undue wear. The larger the difference, the greater the problem. With limited slip differentials the problem will be far worse. For more on how tire size affects speedometer error, see our online calculator.