Formula
Actual speed = indicated speed × new diameter ÷ old diameter
Offroaders.com uses the same ratio.
True speed from a 31.6 in original tire
| New tire | Diameter change | Reads 60 | Reads 70 |
|---|---|---|---|
| 30 in | −5.06% | 57.0 | 66.5 |
| 31 in | −1.90% | 58.9 | 68.7 |
| 32 in | +1.27% | 60.8 | 70.9 |
| 33 in | +4.43% | 62.7 | 73.1 |
| 34 in | +7.59% | 64.6 | 75.3 |
| 35 in | +10.76% | 66.5 | 77.5 |
| 37 in | +17.09% | 70.3 | 82.0 |
| 40 in | +26.58% | 75.9 | 88.6 |
Actual speed = indicated × new diameter ÷ old diameter. 31.6 in is a 265/70R17.
Worked example
A 265/70R17 is 265 × 0.70 = 185.5 mm of sidewall, or 7.30 inches. Two sidewalls plus the 17-inch wheel make 31.6 inches. Moving to a 285/70R17 (32.7 inches) is 3.5% taller: at an indicated 60 mph you are doing 62.1 mph, and engine RPM at a given speed drops 3.4%. To bring the gearing back from 3.73, the ratio would need to be 3.73 × 32.7 ÷ 31.6 = 3.86.
Before you change tire size
- Speedometer and odometer read low by the same percentage the diameter grows.
- Gearing gets taller: engine RPM at a given speed drops by the same percentage, which costs acceleration and towing pull.
- Clearance at full lock and full suspension compression; a wider tire also needs the right wheel width and offset.
- Load and speed ratings must still meet the vehicle's placard.
Common sizes worked out
| Size | Diameter | Width | Revs per mile |
|---|---|---|---|
| 245/75R16 | 30.47 in | 9.6 in | 662 |
| 265/70R17 | 31.61 in | 10.4 in | 638 |
| 275/65R18 | 32.07 in | 10.8 in | 629 |
| 285/70R17 | 32.71 in | 11.2 in | 617 |
| 295/70R18 | 34.26 in | 11.6 in | 589 |
| 315/70R17 | 34.36 in | 12.4 in | 587 |
Calculated from the size code.
Reading a metric tire size
In 285/70R17, 285 is the section width in millimeters, 70 is the sidewall height as a percentage of that width, R means radial and 17 is the wheel diameter in inches. The sidewall is 285 × 0.70 = 199.5 mm (7.85 in); add two sidewalls to the 17-inch wheel and the tire stands about 32.7 inches tall.
Common questions
How do I find my tire's diameter?
Use the size on the sidewall: section width × aspect ratio × 2 ÷ 2,540, plus the wheel diameter. Or measure from the ground to the top of the tread with the vehicle unloaded.
Sources
- https://www.offroaders.com/info/tech-corner/reading/mph-rpm.htm