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.
285/70R18 compared with LT315/70R17
The new size (LT315/70R17) is 1.94% taller than the current size (285/70R18): about 0.7 in (17 mm) more overall diameter.
It is 30 mm wider, with a sidewall 21 mm taller.
The current size is made for a 18-inch wheel and the new size for a 17-inch wheel, so changing between them means different wheels.
| Measurement | Current (285/70R18) | New (LT315/70R17) | Difference (new minus current) |
|---|---|---|---|
| Overall diameter | 33.7 in (856 mm) | 34.4 in (873 mm) | +0.7 in (+17 mm), +1.94% |
| Section width | 11.2 in (285 mm) | 12.4 in (315 mm) | +1.2 in (+30 mm) |
| Sidewall height | 7.9 in (200 mm) | 8.7 in (221 mm) | +0.8 in (+21 mm) |
| Circumference | 105.9 in (2690 mm) | 108.0 in (2742 mm) | +2.1 in (+52 mm) |
| Revolutions per mile | 598 | 587 | -11 |
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 1.94%, 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 taller tire covers more 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 low: when it shows 60 mph, your actual speed is about 61.2 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 | 20.4 mph | +0.4 mph |
| 30 mph | 30.6 mph | +0.6 mph |
| 40 mph | 40.8 mph | +0.8 mph |
| 50 mph | 51.0 mph | +1.0 mph |
| 60 mph | 61.2 mph | +1.2 mph |
| 70 mph | 71.4 mph | +1.4 mph |
| 80 mph | 81.6 mph | +1.6 mph |