Understand the numbers
Tire and wheel calculation guides
Each guide answers a specific question with worked calculations, source references, and a ready-to-use calculator example. Start with the topic behind your size change.
Tire size explained: how to read the numbers
Decode tire width, aspect ratio, rim diameter, and service markings with a worked 225/45R17 example. Learn which numbers the calculator uses.
Tire size conversion: metric sizes and inches
Convert metric tire sizes to inches, see common metric sizes grouped into 31- to 35-inch classes, and read flotation sizes such as 35x12.50R17.
Wheel offset explained: ET, poke, and inner movement
Understand positive and negative wheel offset, see a worked wheel-width example, and distinguish relative edge movement from actual vehicle clearance.
Tire size and speedometer error: how to calculate it
Calculate speedometer error after a tire size change, with a truck example, a percent-to-mph table, and how dealers and programmers correct the reading.
For individual tire measurements, use the tire size chart. For the complete calculation and evidence policy, read our methodology. Open the free tire size calculator when you are ready to compare.