Tire Size Calculator
Compare two tire sizes and see the effect on diameter, speedometer reading and ground clearance.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Compare nominal tire diameters and estimate actual speed when the speedometer reads 60 mph. A diameter difference under 3% is only a common rule of thumb, not a safety approval; verify manufacturer-approved sizes, wheel clearance, load and speed ratings.
Worked example
calculated with the default values below
New tire width 225 mm · New aspect ratio 45 % · New wheel diameter 17 in · Original width 205 mm · Original aspect ratio 55 % · Original wheel diameter 16 in New tire diameter 24.97 in · Original tire diameter 24.88 in · Diameter difference 0.38 % · Speedometer error at 60 mph 0.23 mph · 3% rule of thumb (verify fitment) Within 3% rule of thumb — verify vehicle fitment · Estimated actual speed at 60 mph indicated 60.2 mph
General background references
Disclaimer
These calculators give estimates for general information only. Real fuel consumption, performance and range vary with driving style, load, terrain, temperature and vehicle condition. Do not rely on these results for towing limits, payload or braking decisions — use the rating plate of your vehicle.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
D = R + 2 × W × (a ÷ 100) ÷ 25.4; Δ% = (D − D₀) ÷ D₀ × 100; v = 60 × D ÷ D₀
- D
- New overall diameter (in)
- R
- Rim diameter (in)
- W
- Section width (mm)
- a
- Aspect ratio (%)
- D₀
- Old overall diameter (in)
- v
- Actual speed at indicated 60
Swap is judged safe when |Δ%| ≤ 3; if D₀ = 0 then Δ% = 0 and v = 60.
Frequently asked questions
How is the speedometer estimate calculated?
It assumes indicated speed scales with the ratio of new to original nominal tire diameter: at 60 mph indicated, estimated speed = 60 × new diameter ÷ original diameter. This comparison does not establish fitment or safety approval; check the vehicle maker’s approved sizes and all load, clearance and speed requirements.