Garage tool 04
Tire size decoder
What the numbers on the sidewall mean, and what changing size does to the bike.
Pick a bike to load its stock rear tire into both boxes.
Metric (110/90-19 62M TT) or inch (3.00-18) sizes. Suggestions list every size on the site.
Enter two sizes to compare them.
Diameters are calculated from the size, unloaded. Real tires vary by maker and wear; inch-series sizes are taken as a 100% profile. Only change tire sizes within what your rims, fender clearance and the tire maker allow.
Reading a size
110/90-19: the tire is 110 mm wide, the sidewall is 90% of that width (99 mm), and it fits a 19-inch rim.
62M: load index 62 (265 kg per tire) and speed rating M (up to 130 km/h).
TT means it runs a tube; NHS means not for highway service.
Why diameter matters
A taller rear tire covers more ground per turn, which is the same as gearing up: a 2.7% taller tire acts much like dropping about one tooth from a 50-tooth rear sprocket. Check the effect in the gearing calculator.