Tire size calculator
Enter your current size and the size you’re considering. Everything below updates as you change the numbers — diameter, clearance, speedometer error and whether it fits.
Example: 27x8.00R14 vs 245/40R18. Change either size to compare your own.
Start from your car make, model, year
That size does not exist: the overall diameter must be larger than the rim. The results below still show the last valid pair.
Width in mm · aspect = sidewall height as a % of width · rim in inches
Flotation (LT) sizes, all in inches: overall diameter × section width, then rim
27x8.00R14: 27.00″ (686 mm) tall, 8.00″ (203 mm) wide, sidewall 6.50″ (165 mm) — 747 revolutions per mile.
Outside the 3% rule
−4.75% diameterThis exceeds the tolerance most manufacturers allow. Expect speedometer error, possible rubbing, and on some cars ABS or stability-control faults.
- Needs 18″ wheels: this tire does not fit your 14″ rims.
- 42 mm wider: check inner clearance and that your rims are wide enough.
Check what fits your car Factory tire sizes for every make, model and year.
Tires in 245/40R18
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| Measurement | 27x8.00R14 | 245/40R18 | Diff | % |
|---|---|---|---|---|
| Overall diameter | 27.00″686 mm | 25.72″653 mm | −1.28″ | −4.8% |
| Section width | 203.2 mm | 245.0 mm | +41.8 mm | +20.6% |
| Sidewall height | 165.1 mm | 98.0 mm | −67.1 mm | −40.6% |
| Rim diameter | 14″ | 18″ | +4″ | +28.6% |
| Circumference | 84.82″2155 mm | 80.79″2052 mm | −4.03″ | −4.8% |
| Revolutions per mileturns per distance; drives the odometer | 747464 /km | 784487 /km | +37 | +5.0% |
| Ride height change | — | — | −0.64″ −16.3 mm | |
Speedometer reading
Overlay
How to use the tire size calculator
Put your current tire size — width, aspect ratio and rim diameter — into the first form, then your desired size into the second. You’ll get the difference between the new and the old size, plus the speedometer reading difference and your actual speed on the new set. Both metric and flotation (LT inch) sizes are supported.
The calculator helps you decide whether a set of tires and wheels will fit your car. To compare tires by brand and model instead, use our tire comparison tool; to look up factory sizes, browse by car make.
Changing wheels too? The wheel offset calculator shows how far a new wheel width and offset move the tire toward the suspension and the fender.
Reading a tire size
- 225
- Section width in millimetres
- 45
- Aspect ratio — sidewall height as a % of width
- R
- Radial construction
- 17
- Rim diameter in inches
How the numbers work
How tire diameter is calculated
diameter = rim + 2 × (width × aspect ÷ 100) ÷ 25.4
Multiply the section width by the aspect ratio to get the sidewall height, double it and add the rim. For 225/45R17: 225 × 0.45 = 101.25 mm per sidewall, 202.5 mm (7.97″) for both, plus the 17″ rim = 24.97″ (634 mm).
Flotation sizes skip the maths: 33x12.50R15 is 33″ tall and 12.5″ wide by definition, on a 15″ rim.
The 3% rule
Car makers and tire shops generally accept a replacement size whose overall diameter is within 3% of the original — about 0.75″ (19 mm) on a 25″ tire. Inside that range the speedometer, ABS and gearing barely notice.
Beyond it, check the clearance at full steering lock and full suspension compression, and expect the speedometer to be off by the same percentage.
Speedometer error
Speedometers count wheel turns. A taller tire travels further per turn, so the car goes faster than the speedometer says; a shorter one makes it read high. The error is the same percentage at every speed.
Move the slider in the speedometer card above to see your real speed on the new size.
Tire size calculator FAQ
How do I calculate a tire’s diameter?
Multiply the section width by the aspect ratio to get the sidewall height, double it and add the rim. For 225/45R17: 225 × 0.45 = 101.25 mm per sidewall, 202.5 mm (7.97″) for both, plus the 17″ rim = 24.97″ (634 mm).
Can I put a different tire size on my car?
Usually, if the overall diameter stays within about 3% of the original, the tire suits your rim width and it clears the suspension and wheel arches at full lock. Its load index and speed rating must be at least those of the original tires.
How does tire size affect my speedometer?
The speedometer counts wheel turns. A taller tire covers more ground per turn, so you drive faster than it shows: with a tire 3% taller you are doing about 61.8 mph when it reads 60. A smaller tire makes it read high.
What is the 3% rule?
Most fitment guides accept a new size whose overall diameter is within 3% of the original. Beyond that, speedometer error grows, the tire may rub, and ABS or stability control can be affected.
What does 225/45R17 mean?
225 is the section width in millimetres, 45 the sidewall height as a percentage of that width, R stands for radial construction and 17 is the rim diameter in inches.
How do I convert a metric tire size to inches?
Enter it as your current size: the calculator shows its overall diameter, width and sidewall in inches and millimetres. 285/75R16, for example, is 32.8″ × 11.2″ — about the same as a 33x11.50R16 flotation tire.