Tire size conversion chart
Convert any pair, or read the common ones off the chart below.
| Measurement | New | Difference |
|---|---|---|
| Overall diameter | 31.00 in | +2.12 in (+7.3%) |
| Section width | 10.50 in | +1.25 in |
| Sidewall height | 8.00 in | +1.06 in |
| Circumference | 97.39 in | +6.67 in |
| Revolutions per mile | 651 | −48 |
| Ride height change | +1.06 in | half the diameter change |
| Speedo shows | You are actually doing | Difference |
|---|---|---|
| 20 | 21.5 | +1.5 |
| 30 | 32.2 | +2.2 |
| 40 | 42.9 | +2.9 |
| 50 | 53.7 | +3.7 |
| 60 | 64.4 | +4.4 |
| 70 | 75.1 | +5.1 |
| 80 | 85.9 | +5.9 |
Popular conversions
Every row is computed from published dimensions with the same formulas the converter above uses, not copied from another chart.
| Stock size | New size | New diameter | Difference | Speedo reads | Fitment |
|---|---|---|---|---|---|
| 225/45R17 | 245/40R18 | 25.72 in | +0.74 in (+3.0%) | 66.9 mph | Diameter: 3.0% larger |
| 205/55R16 | 225/45R17 | 24.97 in | +0.09 in (+0.4%) | 65.2 mph | Diameter: 0.4% larger |
| 235/40R18 | 245/35R19 | 25.75 in | +0.35 in (+1.4%) | 65.9 mph | Diameter: 1.4% larger |
| 265/70R17 | 285/70R17 | 32.71 in | +1.10 in (+3.5%) | 67.3 mph | Aggressive: 3.5% larger |
| 245/65R17 | 265/65R17 | 30.56 in | +1.02 in (+3.5%) | 67.3 mph | Aggressive: 3.5% larger |
| 225/65R17 | 235/65R17 | 29.03 in | +0.51 in (+1.8%) | 66.2 mph | Diameter: 1.8% larger |
| 275/60R20 | 285/55R20 | 32.34 in | −0.65 in (−2.0%) | 63.7 mph | Diameter: 2.0% smaller |
| 215/60R16 | 225/55R17 | 26.74 in | +0.59 in (+2.2%) | 66.5 mph | Diameter: 2.2% larger |
| 245/45R18 | 255/40R19 | 27.03 in | +0.35 in (+1.3%) | 65.9 mph | Diameter: 1.3% larger |
| 265/75R16 | 285/75R16 | 32.83 in | +1.18 in (+3.7%) | 67.4 mph | Aggressive: 3.7% larger |
| 195/65R15 | 205/55R16 | 24.88 in | −0.10 in (−0.4%) | 64.7 mph | Diameter: 0.4% smaller |
| 235/75R15 | 31x10.5R15 | 31.00 in | +2.12 in (+7.3%) | 69.8 mph | Not recommended: 7.3% larger |
| 285/45R22 | 305/40R22 | 31.61 in | −0.49 in (−1.5%) | 64.0 mph | Diameter: 1.5% smaller |
| 165/70R14 | 185/60R15 | 23.74 in | +0.65 in (+2.8%) | 66.8 mph | Diameter: 2.8% larger |
| 255/35R20 | 265/30R21 | 27.26 in | +0.23 in (+0.9%) | 65.6 mph | Diameter: 0.9% larger |
| 225/40R18 | 235/35R19 | 25.48 in | +0.39 in (+1.6%) | 66.0 mph | Diameter: 1.6% larger |
Speedometer figures assume the speedometer reads 65 mph with the stock size fitted. Difference values are the true speed minus the indicated speed.
Reading the chart
A positive difference means the new tire is larger, so your speedometer under-reads: you are going faster than it says. A negative difference means the tire is smaller and the speedometer over-reads, which is the safer direction for a mistake but still affects the odometer.
Why a table cannot cover every case
Published tire dimensions are nominal. Two tires marked 285/70R17 from different manufacturers can differ by a few tenths of an inch in actual mounted diameter, and tread wear changes it further over the tire's life. Use the chart to narrow the choice, then confirm clearance on the vehicle.