What changes with a bigger tire
A taller tire covers more ground per turn, so the speedometer reads low by the same percentage as the diameter increase, and ground clearance rises by half the diameter change.
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.
Diameter on a 17 in wheel by width and aspect ratio
| Aspect | 255 mm | 265 mm | 275 mm | 285 mm | 295 mm | 315 mm |
|---|---|---|---|---|---|---|
| 60 | 29.05 | 29.52 | 29.99 | 30.46 | 30.94 | 31.88 |
| 65 | 30.05 | 30.56 | 31.07 | 31.59 | 32.10 | 33.12 |
| 70 | 31.06 | 31.61 | 32.16 | 32.71 | 33.26 | 34.36 |
| 75 | 32.06 | 32.65 | 33.24 | 33.83 | 34.42 | 35.60 |
Overall diameter in inches.
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://gearstar.com/rpm-calculator/
- https://www.offroaders.com/info/tech-corner/reading/mph-rpm.htm