Tire Size Calculator
Enter your current tire size and, to compare, a new size. You get the diameter, the sidewall height, the revolutions per mile and what your speedometer would really read.
Your numbers
How we calculate it
A size such as 225/45R17 gives the section width in millimeters (225), the aspect ratio as a percentage of the width (45) and the rim diameter in inches (17). The sidewall height is width × aspect ratio ÷ 100, and the overall diameter is the rim diameter × 25.4 plus two sidewalls. The circumference is π × the diameter, and the revolutions per mile are one mile (1,609,344 mm) divided by the circumference.
For 225/45R17 the sidewall is 101.25 mm and the diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm (24.97 in), or about 807.6 revolutions per mile. For 235/40R18 the diameter is 645.2 mm, which is 1.72% larger. A speedometer calibrated for the first size then shows 60 when the car is really going 61.0.
Assumptions and limits
- The speedometer comparison assumes the speedometer was set up for tire size 1. A larger tire means you are going faster than the speedometer says, and a smaller one means slower.
- A common piece of advice is to keep the overall diameter within about 3% of the original size. Check the manual for your car as well, because the load rating, the clearance and the ABS or stability systems matter too.
- Sizes are metric passenger-car sizes: width 125 to 395 mm, aspect ratio 20 to 95 and a rim of 10 to 26 inches. A prefix (P, LT) and a load index and speed rating after the size are accepted and ignored. Actual diameters vary a little with the tire brand, the wear and the pressure.
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17, 225 is the width in millimeters, 45 is the height of the sidewall as a percentage of the width, R means radial construction and 17 is the wheel diameter in inches.
How do I calculate tire diameter?
Multiply the rim diameter by 25.4 to get millimeters, then add twice the sidewall height, where the sidewall is the width times the aspect ratio divided by 100.
How much will my speedometer be off with bigger tires?
The error is the difference in diameter. If the new tire is 1.72% larger, a speedometer that reads 60 means the real speed is about 61.0. Enter both sizes to see the figure for your car.
Related calculators
- Speed ConverterEnter a speed, choose the unit it is in and the unit you want.
- Length ConverterEnter a length, choose the unit it is in and the unit you want.
- Gas Mileage CalculatorChoose the units you measured in, then enter the distance you drove and the fuel you used.
- Fuel Economy ConverterEnter a fuel economy figure, choose the unit it is in and the unit you want.
Worked example (a fixed example using the starting numbers, it does not change as you type)
Inputs: Tire size 1 (as on the sidewall, for example 225/45R17) = 225/45R17; Tire size 2 to compare = 235/40R18; Speed shown on the speedometer = 60.
- Size 1 overall diameter 634.3 mm
- Size 1 diameter in inches 24.97 in
- Size 1 sidewall height 101.3 mm
- Size 1 revolutions per mile 807.6
- Size 2 overall diameter 645.2 mm
- Size 2 diameter in inches 25.40 in
- Size 2 sidewall height 94.0 mm
- Size 2 revolutions per mile 794.0
- Diameter difference 1.72 %
- Actual speed at the indicated speed 61.0
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