One of the tire's will have a diameter that is 381mm (15 inches) + 114mm + 114mm = 609mm and the other tire will have a diameter that is 381mm (15 inches) + 120.25mm + 120.25mm = 621.5mm. The difference in diameters will be 12.5mm. Since circumference is Pi X diameter, the difference in circumference is 12.5 x 3.14 = 39.25mm. This will make a roughly 2% difference. So if you are going at 55 mph, your speedometer will show 56.1mph.

John
Our HT