Tire size calculator
Compare two tire sizes side by side: overall diameter, width, sidewall height, circumference and revolutions per mile, plus how your speedometer reading changes. Type sizes in any common format.
255/55R20 compared with 315/40R21
The new size (315/40R21) is 0.39% shorter than the current size (255/55R20): about 0.1 in (3 mm) less overall diameter.
It is 60 mm wider, with a sidewall 14 mm shorter.
The current size is made for a 20-inch wheel and the new size for a 21-inch wheel, so changing between them means different wheels.
| Measurement | Current (255/55R20) | New (315/40R21) | Difference (new minus current) |
|---|---|---|---|
| Overall diameter | 31.0 in (789 mm) | 30.9 in (785 mm) | -0.1 in (-3 mm), -0.39% |
| Section width | 10.0 in (255 mm) | 12.4 in (315 mm) | +2.4 in (+60 mm) |
| Sidewall height | 5.5 in (140 mm) | 5.0 in (126 mm) | -0.6 in (-14 mm) |
| Circumference | 97.5 in (2477 mm) | 97.1 in (2467 mm) | -0.4 in (-10 mm) |
| Revolutions per mile | 650 | 652 | +3 |
These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.
Is the difference small?
The overall diameter differs by 0.39%, which is within the common 3% rule of thumb.
The rule of thumb keeps a replacement size within about 3% of the original overall diameter. It covers diameter only. It is not a fit, load or safety check, because width, wheel diameter, load capacity and clearance matter too. Confirm any size change with your vehicle's owner's manual or a tire professional.
Speedometer difference
A shorter tire covers less ground on every turn, but the speedometer counts turns as if the tire were still the original size. With the new size the speedometer reads high: when it shows 60 mph, your actual speed is about 59.8 mph. These figures assume the speedometer is accurate on the current size and use nominal tire dimensions, so treat them as estimates.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 19.9 mph | -0.1 mph |
| 30 mph | 29.9 mph | -0.1 mph |
| 40 mph | 39.8 mph | -0.2 mph |
| 50 mph | 49.8 mph | -0.2 mph |
| 60 mph | 59.8 mph | -0.2 mph |
| 70 mph | 69.7 mph | -0.3 mph |
| 80 mph | 79.7 mph | -0.3 mph |