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: 205/65R14 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
Outside the 3% rule
+5.00% 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.
- 40 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 | 205/65R14 | 245/40R18 | Diff | % |
|---|---|---|---|---|
| Overall diameter | 24.49″622 mm | 25.72″653 mm | +1.22″ | +5.0% |
| Section width | 205.0 mm | 245.0 mm | +40.0 mm | +19.5% |
| Sidewall height | 133.3 mm | 98.0 mm | −35.3 mm | −26.5% |
| Rim diameter | 14″ | 18″ | +4″ | +28.6% |
| Circumference | 76.94″1954 mm | 80.79″2052 mm | +3.85″ | +5.0% |
| Revolutions per mileturns per distance; drives the odometer | 823512 /km | 784487 /km | −39 | −4.8% |
| Ride height change | — | — | +0.61″ +15.5 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.
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