Tire Size Calculator
Compare two tire sizes side by side: overall diameter, width, sidewall, circumference, revolutions per kilometre — and how far your speedometer will be off, in km/h.
Start from your vehicle
Optional — pick your car and we fill in the factory size. Or type the sizes below.
Front
| Size 1 (current) | Size 2 (new) | Difference | |
|---|---|---|---|
| Diameter | – | – | |
| Width | – | – | |
| Sidewall | – | – | |
| Circumference | – | – | |
| Revs / km | – | – |
Size comparison
Speedometer error with Size 1 set to your factory size
| Reading (km/h) | 30 | 40 | 50 | 60 | 80 | 100 | 120 | 140 |
|---|---|---|---|---|---|---|---|---|
| Actual (km/h) | – | – | – | – | – | – | – | – |
Rear
| Size 1 (current) | Size 2 (new) | Difference | |
|---|---|---|---|
| Diameter | – | – | |
| Width | – | – | |
| Sidewall | – | – | |
| Circumference | – | – | |
| Revs / km | – | – |
Size comparison
Speedometer error with Size 1 set to your factory size
| Reading (km/h) | 30 | 40 | 50 | 60 | 80 | 100 | 120 | 140 |
|---|---|---|---|---|---|---|---|---|
| Actual (km/h) | – | – | – | – | – | – | – | – |
Estimates from tire geometry. Brakes, fender liners, suspension travel and the tire's actual make are not modelled — we confirm every fitment on the car before we quote it.
Changing wheels too? Compare offset and poke with the wheel size calculator →
How to use it
- Pick your vehicle (optional) — Size 1 fills with the factory size, the one printed on the driver's door-jamb placard. No match? Just type it.
- Type the size you are considering as Size 2. On a car with different front and rear sizes, tick "Staggered" and each axle gets its own comparison.
- Read the Difference column and the speedometer table. Diameter is the number that matters most.
Reading the numbers
Sidewall height is width × aspect ratio. Two sidewalls plus the rim make the overall diameter, and that one number sets circumference, revs per kilometre and what your speedometer believes. A taller tire turns fewer times per kilometre, so the gauge reads low; a shorter one reads high. Within about 3% almost every car is fine; beyond that, ABS, stability control and the odometer are all working from bad numbers, and the tire may touch the liner at full lock.
Winter sets often go one rim size down with a taller sidewall — the diameter still matches. See our winter tire & wheel packages built that way for your car, or read how to read a tire size.
Common questions
How much can a replacement tire size differ from the factory size?
Keep the overall diameter within about 3% of the factory size — that is what the speedometer, ABS and stability control are calibrated to — and keep the load index at or above the placard value. Inside that window most cars are fine; beyond it, ask before buying. The size on the door-jamb placard is always the safe default.
Why does a different tire size change my speedometer?
The car counts wheel revolutions and assumes each one covers the factory circumference. A taller tire covers more ground per turn, so the gauge reads LOW (you are going faster than it says); a shorter tire reads high. The speedometer table above shows the true speed at each reading once Size 1 is set to your factory size.
Can I go down a rim size for winter tires?
Yes — this is called minus-sizing and it is common for winter sets: a smaller rim with a taller sidewall, chosen so the overall diameter stays within a percent of stock. Narrower winter tires also cut through slush better. Use the calculator to confirm the diameter, and check brake caliper clearance on the smaller rim — we do this for every winter package we build.
What does "revs per km" mean?
How many times the tire turns to cover one kilometre — one million millimetres divided by the circumference. ABS, traction control and the odometer all work from this number, which is why a large change in diameter upsets them.
Want us to check it on your car?
Send us the sizes you compared and your vehicle — we confirm the fitment and email an all-in installed price.