Tire size comparison calculator
Enter your current tire size and the size you are considering. You get the full picture instantly: diameter change, sidewall and width differences, revolutions per mile, and how far your speedometer will be off.
| Measurement | Current | New | Difference |
|---|
Speedometer error
Your speedometer is calibrated for the current tire. With the new tire fitted, this is how fast you are really going when the dial shows:
| Speedometer shows | Actual speed | Error |
|---|
The short answer
Two tires are interchangeable when their overall diameters are close. The widely used rule of thumb is to stay within ±3% of the original diameter: beyond that, the speedometer error grows past what most drivers accept, gearing and clearance start to matter, and on some vehicles the ABS and stability systems notice. Diameter is worked out from the size code alone — rim diameter in inches × 25.4, plus twice the sidewall height (tire width × aspect ratio ÷ 100). Everything else on this page — circumference, revolutions per mile, speedometer error — follows from that one number.
Flotation sizes such as 31x10.50R15 skip the arithmetic: the first number already is the overall diameter in inches and the second is the width, so the sidewall is simply (31 − 15) ÷ 2 = 8 inches. This calculator reads both formats, including an optional P or LT prefix.
Popular comparisons
Each page has the full table for that pair, worked out in advance:
How the numbers are calculated
| Sidewall height | width × aspect ratio ÷ 100 |
| Overall diameter | rim (in) × 25.4 + 2 × sidewall |
| Circumference | π × diameter |
| Revolutions per mile | 63,360 ÷ diameter (in) |
| Speedometer error | shown speed × (new diameter ÷ old diameter) |
Figures come from the size code and these standard formulas, not from any manufacturer's catalogue. Real tires of the same nominal size vary slightly by brand and model, and a worn tire is measurably smaller than a new one. Treat results as accurate to the code, and confirm fitment against your vehicle manual or a professional tire shop before buying.
Frequently asked questions
How much diameter difference is acceptable when changing tire size?
The common guideline is within ±3% of the original overall diameter. Inside that band, speedometer error stays under about 3% and clearance or gearing problems are unlikely on most vehicles. It is a rule of thumb, not a guarantee — your vehicle manual and a professional tire shop have the final say.
Why does my speedometer read wrong after fitting bigger tires?
The speedometer counts wheel revolutions and assumes the original tire's circumference. A taller tire travels further per revolution, so at the same indicated speed you are actually going faster than the dial shows — by exactly the diameter ratio between the two sizes.
What does 265/70R17 actually mean?
265 is the tread width in millimetres, 70 is the sidewall height as a percentage of that width, R means radial construction, and 17 is the wheel diameter in inches. Our guide to reading a tire size walks through every part of the sidewall, including load index and speed rating.
Can I compare a metric size with a flotation size like 33x12.50R18?
Yes. Type either format in either box — for example 275/65R18 against 33x12.50R18 — and the calculator converts both to the same diameter, width and sidewall figures before comparing.
Does a wider tire change the speedometer?
Only through diameter. Width on its own does not affect the speedometer; what matters is the overall diameter, which width influences only when the aspect ratio stays the same (a wider tire at the same aspect ratio has a taller sidewall, and therefore a larger diameter).