How to read a tire size
A tire size such as P265/70R17 113T breaks down like this:
| Part | Meaning |
|---|---|
| P | Tire type: P for passenger, LT for light truck. Euro-metric sizes have no letter. |
| 265 | Section width in millimeters, sidewall to sidewall. |
| 70 | Aspect ratio: the sidewall height is 70% of the width. |
| R | Radial construction. |
| 17 | Rim diameter in inches. |
| 113T | Load index and speed rating. |
Off-road and light-truck tires are often sold in flotation sizes such as 33x12.50R17: 33 inches tall, 12.5 inches wide, on a 17 inch wheel. The calculator accepts both formats.
How tire diameter is calculated
Sidewall = width × aspect ratio ÷ 100
Diameter = rim × 25.4 + 2 × sidewall (in mm)
Revolutions per mile = 63,360 ÷ (π × diameter in inches)
Example. 265/70R17: the sidewall is 265 × 0.70 = 185.5 mm. The diameter is 17 × 25.4 + 2 × 185.5 = 802.8 mm, or 31.6 inches. These are nominal sizes; actual tires vary slightly by brand and shrink a little as the tread wears.
Tire size chart in inches
| Size | Diameter | Width | Sidewall | Revs per mile |
|---|---|---|---|---|
| 205/55R16 | 24.9" | 8.1" | 4.4" | 811 |
| 215/60R16 | 26.2" | 8.5" | 5.1" | 771 |
| 225/45R17 | 25.0" | 8.9" | 4.0" | 808 |
| 225/65R17 | 28.5" | 8.9" | 5.8" | 707 |
| 235/65R17 | 29.0" | 9.3" | 6.0" | 695 |
| 235/60R18 | 29.1" | 9.3" | 5.6" | 693 |
| 245/45R18 | 26.7" | 9.6" | 4.3" | 756 |
| 255/55R18 | 29.0" | 10.0" | 5.5" | 694 |
| 265/70R16 | 30.6" | 10.4" | 7.3" | 659 |
| 265/70R17 | 31.6" | 10.4" | 7.3" | 638 |
| 265/65R18 | 31.6" | 10.4" | 6.8" | 639 |
| 275/60R20 | 33.0" | 10.8" | 6.5" | 611 |
| 275/55R20 | 31.9" | 10.8" | 6.0" | 632 |
| 275/65R18 | 32.1" | 10.8" | 7.0" | 629 |
| 275/70R18 | 33.2" | 10.8" | 7.6" | 608 |
| 285/70R17 | 32.7" | 11.2" | 7.9" | 617 |
| 285/75R16 | 32.8" | 11.2" | 8.4" | 614 |
| 285/65R18 | 32.6" | 11.2" | 7.3" | 619 |
| 305/70R17 | 33.8" | 12.0" | 8.4" | 596 |
| 315/70R17 | 34.4" | 12.4" | 8.7" | 587 |
Metric equivalents of 31", 33" and 35" tires
| Height | Metric sizes within about half an inch |
|---|---|
| 31" | 245/75R17, 255/70R17 |
| 32" | 265/70R17, 265/75R16, 275/65R18, 275/55R20, 285/55R20 |
| 33" | 285/70R17, 285/75R16, 275/70R18, 285/65R18, 275/60R20, 295/55R20 |
| 34" | 305/70R17, 315/70R17, 295/70R18, 315/65R18, 295/60R20, 305/60R20 |
| 35" | 315/75R16, 315/70R18, 325/65R18, 295/65R20, 325/60R20 |
| 37" | 315/75R18, 325/65R20, 335/65R20, 315/70R20 |
Speedometer error with bigger tires
Your speedometer counts wheel revolutions. A taller tire covers more ground per turn, so you travel faster than the speedometer shows. The actual speed equals the indicated speed times the new diameter divided by the old one. Going from 265/70R17 to 285/70R17 is a +3.5% change: at an indicated 60 mph you are really doing 62.1 mph, and the odometer under-counts by the same share. Many modern vehicles can be recalibrated with a dealer tool or a programmer.
How much can tire size change?
A common rule of thumb among tire shops is to keep the overall diameter within about 3% of the original, which keeps the speedometer, ABS and stability control close to their calibration. Larger jumps usually need a lift or leveling kit, trimming, recalibration, or new axle gears. Always check the load index and speed rating meet your vehicle's placard on the driver's door jamb.
Regearing after a tire size change
Taller tires make the effective gear ratio taller, so the engine works harder from a stop and on hills. To get the original feel back, multiply your current axle ratio by the new diameter divided by the old diameter. For 265/70R17 to 285/70R17 with 3.73 gears that is 3.86, so the nearest common ratio, 3.91, would be the choice. Enter your ratio above to calculate it.
Will the new tires fit?
The table shows how much taller and wider the new tire is. Half the diameter increase is the extra ground clearance and the extra space needed above the tire. On the same wheel, half the width increase goes inward toward the suspension and half outward toward the fender. Check clearance at full steering lock and with the suspension compressed before buying.