If your speedometer has been calibrated for 140/80 R 15 (24 x 5.51 x 15) tires, replacing them with 140/90 R 15's (24 x 5.51 x 15's) would result in the speedometer reading low by 4.42%.
i.e. you are going 4.42 % faster than indicated at all speeds.
This link is the only tire calculator I found that allows motorcycle sizes.
http://www.dakota-truck.net/TIRECALC/tirecalc.htmlEnter 14/80 - 15 in Tire A and 140/90 - 15 in Tire B.
Select "Compare Tire A to Tire B" and click on Submit.
Speedometer error listed from 5 mph to 200 mph in 5 mph increments.
Quote:Actual Speed Indicated Speed with Tire B Speedometer Error
20 mph (32.18 kph) 19.12 mph (30.76 kph) -0.88 mph (-1.42 kph)
60 mph (96.54 kph) 57.35 mph (92.27 kph) -2.65 mph (-4.27 kph)
65 mph (104.58 kph) 62.12 mph (99.96 kph) -2.88 mph (-4.63 kph)
80 mph (128.72 kph) 76.46 mph (123.03 kph) -3.54 mph (-5.69 kph)
The faster you go, the bigger MPH the difference is.