Decode tire size labels like 225/45R17 and compare alternate fitments.
Overview
Tire labels such as 225/45R17 encode width, aspect ratio, and rim diameter. Section width is in millimeters; aspect ratio is sidewall height as percent of width; R means radial construction with rim in inches.
Overall diameter ≈ rim diameter (converted to mm) + 2 × sidewall, where sidewall = width × aspect/100. Changing diameter affects speedometer accuracy and clearance to fenders and brakes.
Plus-sizing keeps diameter similar while widening tread and lowering sidewall percentage. Always verify load index, speed rating, and manufacturer fitment guidance beyond diameter math.
Compare old versus new tire diameters to estimate speedometer error percent: (new−old)/old × 100%. A larger tire makes the speedometer read slower than actual speed.
Steps
Width/aspect/R format.
Alternate tire to compare.
Width × aspect ÷ 100 per side.
Convert rim inches to mm and add sidewalls.
Percent diameter change.
Confirm physical fit with vehicle specs.
Math
Diameter ≈ Rim_in×25.4 + 2×(Width_mm×Aspect/100)
Sidewall height on both sides plus rim converts overall rolling diameter.
Worked cases
Sidewall 101.25 mm each side
Overall diameter ≈ 634 mm (24.97 in).
Similar diameter target with lower aspect
Check speedometer delta under 3% often desired.
Actual speed ≈ indicated × 1.02
Slightly faster than displayed.
Why use this
Questions