90/90-R19 tires are 1.29 inches (32.8 mm) smaller in diameter than 80/90-R21 tires and the speedometer difference is 4.8% 90/90-R21 tires are 0.71 inches (18 mm) larger in diameter than 90/80-R21 tires and the speedometer difference is 2.6% 100/100-R21 tires are 1.5 inches (38 mm) larger in diameter than 90/90-R21 tires and the speedometer difference is 5.2%. U t = U 100°F (1 + 0.000061 (t - 100)) (1) where. U t = Saybolt Second Universal (SSU) viscosity at temperature t (°F) U 100°F = Saybolt Second Universal (SSU) viscosity at 100°F in Saybolt Universal seconds equivalent to kinematic viscosity in centiStokes at temperature t (°F) Download and print Kinematic Viscosity Unit Converting Chart. 70/100-R21 tires are 0.79 inches (20 mm) smaller in diameter than 80/100-R21 tires and the speedometer difference is 2.9%. The tables below were created to be a guide for determining how large a wheel/tire size one can fit into a stock motorcycle withough having to undergo modification. Note: Rotor and pulley fitment tables are available here: Rotor Fitment Chart | Pulley Fitment Chart Touring Year Front Wheel Rear Wheel Standard (Harley) DgG3.

80 100 vs 90 90 21