Tire Size Calculator
Compare two tire sizes: diameter, circumference and speedometer error.
Results update as you type.
About this calculator
A tire size calculator compares two tires written in the standard P-metric format (for example 205/55R16) — the section width in millimetres, the aspect ratio as a percentage of that width, and the rim diameter in inches. It computes each tire’s overall diameter as the rim plus twice the sidewall height, the rolling circumference, and the percentage difference between the two.
Worked example: a 205/55R16 has a sidewall of 205 × 0.55 = 112.75 mm, so its diameter is 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm. A 225/45R17 has a 101.25 mm sidewall and a diameter of 431.8 + 202.5 = 634.3 mm — about 0.4% taller.
Because a speedometer is geared to the original tire, fitting a taller or shorter tire makes the reading wrong in proportion to the diameter change; at an indicated 60 mph the taller tire above means a true speed near 60.2 mph. Keep the difference under about 3% to stay within speedometer tolerance and avoid rubbing or gearing problems — which is exactly what drivers use this for when plus-sizing wheels or fitting winter tires.
Frequently asked questions
How much tire size difference is safe?
A change in overall diameter under about 3% keeps the speedometer within tolerance and avoids rubbing or gearing problems. The calculator reports the exact percentage so you can judge a swap.
How does a bigger tire affect my speedometer?
The speedometer counts wheel revolutions against the original tire size. A taller tire travels farther per turn, so you are actually going faster than the dial shows; a shorter tire does the reverse. The calculator gives the true speed at an indicated 60 mph.
What do the numbers in 205/55R16 mean?
205 is the tread width in millimetres, 55 is the sidewall height as a percentage of that width (so 55% of 205 ≈ 113 mm), R means radial construction, and 16 is the wheel rim diameter in inches.
What is plus-sizing a tire?
Plus-sizing fits a larger-diameter wheel with a lower-profile tire so the overall diameter stays about the same — a “plus one” or “plus two” swap. Keeping the total diameter within roughly 3% of stock preserves speedometer accuracy and gearing while changing the look and handling.
If I fit a bigger rim, how do I keep the same overall diameter?
Lower the tire’s aspect ratio (a thinner sidewall) and often widen it slightly to compensate. Moving from a 16-inch to a 17-inch rim, dropping the profile from 55 to 45 keeps the diameter almost identical, as the worked example shows.
Will a taller tire make my speedometer read high or low?
A taller tire travels farther per revolution, so you are actually going faster than the dial shows — the speedometer reads low. A shorter tire is the reverse, reading high. The calculator gives the true speed at an indicated 60 mph for the swap.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/tire-size/curl
curl "https://calculator.free/api/v1/tire-size/?w1=205&a1=55&r1=16&w2=225&a2=45&r2=17"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/tire-size/?" + new URLSearchParams({
"w1": "205",
"a1": "55",
"r1": "16",
"w2": "225",
"a2": "45",
"r2": "17"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.