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Tyre Size Calculator
Use this free tyre size calculator to compare two tyres side by side. Enter the width, aspect ratio and rim size of your current tyre and the one you are thinking of fitting, and it shows the diameter, circumference, sidewall height, revolutions per mile, and how far out your speedometer will read. (Tyre is the usual spelling outside North America; in the United States it is spelled "tire" — the sizes and the maths are identical.)
Current tyre (old)
New tyre
How to read a tyre size
Every tyre has its size printed on the sidewall, in a format like 205/55R16. The first number, 205, is the width of the tyre in millimetres, measured from one sidewall to the other. The second number, 55, is the aspect ratio: the height of the sidewall as a percentage of the width. The letter R means the tyre has radial construction, which almost all modern tyres do. The last number, 16, is the diameter of the wheel rim in inches — the tyre must match this exactly, because a 17-inch tyre simply will not fit a 16-inch wheel.
The sidewall height is the width multiplied by the aspect ratio: on a 205/55 tyre it is 205 × 55% = 112.75 mm, and on a 225/45 tyre it is 225 × 45% = 101.25 mm. A lower aspect ratio means a shorter, stiffer sidewall — sharper steering but a firmer ride; a higher ratio means a taller sidewall and a softer ride over bumps.
How the comparison works
The overall diameter is what matters most, and it comes from the rim plus two sidewalls — one at the top of the wheel and one at the bottom. The formula is diameter = rim diameter in mm + 2 × sidewall height. The rim diameter in inches is converted to millimetres by multiplying by 25.4. So a 16-inch rim is 406.4 mm, and with two 112.75 mm sidewalls the total diameter of a 205/55R16 tyre is 406.4 + 225.5 = 631.9 mm.
From the diameter, the rest follows. The circumference — the distance the tyre covers in one full turn — is π × diameter. Revolutions per mile is 63,360 (inches in a mile) divided by the circumference in inches. A larger tyre turns fewer times per mile, which is why it changes the speedometer reading: the speedometer counts wheel turns and assumes the original size, so a bigger tyre makes the car travel further per turn than it expects.
The percentage change in diameter is the key number. If the new tyre is 0.38% larger, then at any speed the real speed is 0.38% higher than the speedometer shows — at 60 mph indicated, the real speed is 60 × 1.0038 = 60.2 mph. A smaller tyre does the opposite: the real speed is lower than the dial shows. Most car makers and tyre fitters recommend keeping the diameter within about 3% of the original, because a bigger change also affects the gearing, the odometer, wheel-arch clearance, and in some cars the anti-lock braking and traction control.
Changing width alone also matters. A wider tyre gives more grip, but it can rub on the suspension or the inside of the wheel arch at full steering lock, and it usually raises fuel consumption slightly. It also needs a rim wide enough to support it. Before buying, check the load and speed ratings on the new tyre — the load rating must match or beat the original, and the speed rating must suit the car.
Worked example
A driver wants to swap from 205/55R16 tyres to 225/45R17 tyres. Are they a safe match, and what happens to the speedometer?
Step 1: Old tyre sidewall — 205 × 55% = 112.75 mm.
Step 2: Old tyre diameter — 16 inches × 25.4 = 406.4 mm rim, plus 2 × 112.75 = 225.5 mm of sidewall, giving 631.9 mm.
Step 3: New tyre sidewall — 225 × 45% = 101.25 mm.
Step 4: New tyre diameter — 17 inches × 25.4 = 431.8 mm rim, plus 2 × 101.25 = 202.5 mm of sidewall, giving 634.3 mm.
Step 5: Difference — 634.3 − 631.9 = 2.4 mm, which is 2.4 ÷ 631.9 × 100 = +0.38%. That is well inside the usual 3% limit, so the swap is a good match.
Step 6: Speedometer — at 60 mph indicated, the real speed is 60 × (634.3 ÷ 631.9) = 60.2 mph — only 0.2 mph low, which is negligible.
For comparison, the circumferences are 1,985.2 mm (old) and 1,992.7 mm (new), and revolutions per mile are 804 (old) and 801 (new) — the new tyre turns 3 fewer times per mile, which is why the car travels slightly further per turn.
FAQ
How do I read my tyre size?
The size is printed on the tyre sidewall, for example 205/55R16. The first number, 205, is the width in millimetres. The second number, 55, is the aspect ratio — the sidewall height as a percentage of the width. The R means radial construction, and the last number, 16, is the rim diameter in inches. Enter those three numbers into the calculator to compare sizes.
How much bigger can my new tyres be?
As a general rule, keep the overall diameter within about 3% of the original tyre — most tyre fitters and car makers recommend staying within 2 to 3%. A bigger change alters the speedometer reading, the gearing, and the clearance in the wheel arch, and the tyre may rub on the bodywork when turning or going over bumps. Always check that the new tyre clears the suspension and arches, and that the load rating matches or beats the original.
Will bigger tyres make my speedometer wrong?
Yes, if the overall diameter changes. A larger tyre covers more ground per turn, so the car travels faster than the speedometer shows. A tyre 3% larger in diameter makes the real speed about 3% higher — 60 mph on the dial becomes about 61.8 mph on the road. A smaller tyre does the opposite. This calculator shows the exact error at 60 mph for your two sizes.
What is the aspect ratio on a tyre?
The aspect ratio is the height of the tyre sidewall expressed as a percentage of the tyre width. On a 205/55 tyre, the sidewall is 55% of 205 mm, which is about 113 mm tall. A lower number, like 45, means a shorter, stiffer sidewall and sharper handling but a firmer ride. A higher number, like 65, means a taller sidewall and a softer, more comfortable ride.
Do I need new wheels when I change tyre size?
Only if the rim diameter changes. The last number in the size, such as the 16 in 205/55R16, must match the wheel it sits on — a 17-inch tyre will not fit a 16-inch wheel. You can often change the width and aspect ratio on the same wheel, as long as the new width suits the rim width and the overall diameter stays close to the original.