What the numbers on a tire mean
Every tire carries a size code such as 225/60R16 on its sidewall. Those three numbers fix the tire’s overall diameter, which in turn decides how far the car travels with each turn of the wheel. Change the diameter and you change what the speedometer reads, how the gearing feels and whether the tire fits in the wheel well. This calculator decodes two sizes and shows exactly how they differ.
Reading the size
In 225/60R16:
- 225 is the section width in millimeters.
- 60 is the aspect ratio: the sidewall is 60 percent as tall as the tire is wide.
- R means radial construction.
- 16 is the wheel diameter in inches.
The mixed units are a historical quirk, so the calculation converts everything to inches.
The formulas
- Sidewall height = width × aspect ratio ÷ 100, converted from millimeters to inches by dividing by 25.4.
- Overall diameter = rim diameter + 2 × sidewall height.
- Difference = new diameter ÷ current diameter − 1.
- Actual speed = speedometer reading × new diameter ÷ current diameter.
- Revolutions per mile = 63,360 inches ÷ (π × diameter).
Worked example: 225/60R16 to 235/55R17
- Current sidewall: 225 × 0.60 = 135 mm = 5.31 in. Diameter: 16 + 2 × 5.31 = 26.63 in.
- New sidewall: 235 × 0.55 = 129.25 mm = 5.09 in. Diameter: 17 + 2 × 5.09 = 27.18 in.
- Difference: 27.18 ÷ 26.63 − 1 = +2.05 percent, within the usual 3 percent guideline.
- When the speedometer shows 60 mph, you are really doing 61.2 mph.
- Revolutions per mile drop from 757 to 742.
Other aspect ratios on a 17-inch wheel
| New size | Diameter difference | New diameter | Actual speed at 60 mph |
|---|---|---|---|
| 235/45R17 | −4.89% | 25.33 in | 57.1 mph |
| 235/50R17 | −1.42% | 26.25 in | 59.1 mph |
| 235/55R17 | +2.05% | 27.18 in | 61.2 mph |
| 235/60R17 | +5.53% | 28.10 in | 63.3 mph |
| 235/65R17 | +9.00% | 29.03 in | 65.4 mph |
Before you change sizes
- Stay within about 3 percent of the original diameter. Larger changes affect the speedometer, odometer, ABS, traction control and automatic transmission shift points.
- Check clearance. A wider or taller tire can rub on the struts, inner fender liners or brake calipers at full steering lock or under load.
- Match the load index and speed rating of the original tires, which you will find on the placard inside the driver’s door jamb.
- Keep all four tires the same size on all-wheel-drive vehicles, where small differences in diameter can strain the center differential.
Speedometers are rarely perfect to begin with, so if accuracy matters to you, compare the reading with a GPS app on a steady stretch of highway after the change.