About this tool
Tire Size Comparison Calculator is a free, in-browser tool that compares two tire sizes so you can see exactly what changes when you plus-size, downsize or fit a different fitment. Enter each tire either as a metric string like 225/45R17 or as three separate fields — section width in millimetres, aspect ratio percent and rim diameter in inches. Nothing is uploaded; every figure is computed locally as you type.
Each tire's sidewall height is width × aspect ratio ÷ 100 (mm). Overall diameter is rim × 25.4 + 2 × sidewall (mm), which converts to inches by ÷ 25.4; circumference is π × diameter and revolutions per mile is 63360 ÷ circumference. The speedometer error is (new diameter − old diameter) ÷ old diameter × 100. A taller second tire makes the speedometer read slower than your true speed, because the wheel is calibrated for the original rolling distance.
Use it to keep your overall diameter within a percent or two of stock so gearing, ABS and the speedometer stay accurate. The tiles show both tires' diameters plus the change in diameter, sidewall, width, circumference and revs per mile, with the speedometer error highlighted first.