Tire Size Comparison Calculator
Compare two tire sizes (e.g. 225/45R17 vs 235/40R18) using the standard tire-geometry formula. Fast, free, and fully client-side.
Calculator Inputs
How the Tire Size Comparison Calculator Works
Compares the overall diameter of two tire sizes given in standard sidewall notation (width/aspect-ratio R rim-diameter), so you can check whether a replacement or upsized tire keeps your vehicle’s effective rolling diameter close to stock.
Formula
Diameter (in) = ((Width (mm) × (Aspect Ratio / 100) × 2) / 25.4) + Rim Diameter (in)
The sidewall height is width × aspect-ratio-percentage, doubled for top and bottom, converted from millimeters to inches, then added to the rim diameter to get the tire’s total outer diameter.
Calculation Example
A 225/45R17 tire: ((225 × 0.45 × 2) / 25.4) + 17 ≈ 8.0 + 17 = 25.0 in diameter.
Frequently Asked Questions
How much diameter difference is safe when swapping tire sizes?
Keep the difference within about 3%. A larger difference can throw off your speedometer, odometer, and ABS/traction-control calibration, since those systems assume a specific wheel-rotation-to-distance ratio.
What do the numbers in a tire size like 225/45R17 mean?
225 is the tire width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R indicates radial construction, and 17 is the rim diameter in inches.
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