VinMole

Tire Size Comparison Calculator

Enter two tire sizes to compare overall diameter, sidewall height, speedometer error, and more — calculated instantly in your browser. No data leaves your device.

Tire A — Current
Tire B — New

All calculations happen in your browser — no tire size data is sent to any server.

Tire Specifications
Comparison
Visual Scale
Speedometer Error

Actual speed when speedometer indicates the values below. Based solely on diameter ratio — other factors (tyre pressure, load, wear) also affect rolling circumference.

These calculations cover geometric differences only. Actual fitment also depends on wheel width, offset (ET), suspension geometry, wheel-arch clearance, and vehicle manufacturer recommendations. Before fitting, verify compatibility with a tyre specialist — or look up your vehicle by VIN on VinMole to see OEM spec data.

How the tire size comparison calculator works

  1. Parse the size string — type a size like "225/45R17" into the text field and the tool fills width, aspect ratio, and rim diameter automatically.
  2. Calculate sidewall height — section width (mm) × aspect ratio ÷ 100. For 225/45: 225 × 0.45 = 101.25 mm.
  3. Calculate overall diameter — rim diameter × 25.4 mm + 2 × sidewall height. For 225/45R17: 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm.
  4. Calculate circumference — π × diameter. For 634.3 mm: 1,992.7 mm.
  5. Calculate revolutions per mile/km — 1,609,344 ÷ circumference(mm) per mile; 1,000,000 ÷ circumference(mm) per km.
  6. Speedometer correction — actual speed = indicated speed × (Tire B diameter ÷ Tire A diameter).

Frequently asked questions

Does changing tire size affect the speedometer?

Yes. A larger overall diameter means the tire travels farther per revolution, so the speedometer — which counts wheel revolutions — reads lower than your actual speed. A smaller diameter has the opposite effect: the speedometer reads higher than actual. This calculator shows the exact difference at common road speeds.

What does 225/45R17 mean?

The three numbers describe the tyre's cross-section. 225 is the section width in millimetres (the tyre's widest point). 45 is the aspect ratio — the sidewall height expressed as a percentage of the section width (225 × 0.45 = 101.25 mm sidewall). R indicates radial construction. 17 is the wheel rim diameter in inches. Combining these three values gives you the tyre's overall diameter.

How much tire size difference is acceptable?

A commonly cited guideline in the automotive community is to stay within ±3% of the original tire's overall diameter. Exceeding this threshold can make speedometer error noticeable, may affect ABS and traction-control calibration (which rely on accurate wheel-speed signals), and can cause clearance issues. This tool highlights any comparison that exceeds that threshold. Always verify fitment against your vehicle manufacturer's specifications.

What is the tire size difference calculator used for?

Common uses include: evaluating a plus-sizing upgrade (larger wheel with lower-profile tyre), checking whether a winter tyre option maintains the same rolling diameter as the OEM summer tyre, confirming that two axles carry matching diameters on all-wheel-drive vehicles, and estimating how much a change will shift ride height.

Can I use this as a speedometer error calculator?

Yes — the speedometer error table is part of this tool. It shows your actual speed at indicated 30, 45, 60, and 75 mph (and the km/h equivalents), based on the diameter ratio between Tire A and Tire B. The calculation assumes a rigid comparison at sea-level tyre pressures; real-world rolling radius also varies with load and inflation.