Tire Size Calculator
Compare tire sizes. Calculate diameter, circumference, and speedometer difference. Free tire calculator.
Reading Tire Size Numbers
A tire marked 225/55R17 means: 225 = width in mm. 55 = aspect ratio (sidewall height is 55% of width). R = radial construction. 17 = rim diameter in inches. The overall diameter determines speedometer accuracy — changing tire size without recalibrating affects your indicated speed and odometer accuracy.
Safe Tire Size Changes
Keep overall diameter within 3% of original to maintain speedometer accuracy and avoid clearance issues. Going wider improves grip but increases road noise and fuel consumption. Going from 55 to 50 aspect ratio (plus one inch rim size) is the most common upgrade — maintains diameter while improving handling. Always verify fitment with your vehicle manufacturer specifications.
Consumer Reports found that at 4/32 inch tread remaining (halfway worn), wet braking distance increases 30%. At 2/32 inch (legal minimum), stopping distance nearly doubles vs new tires.
