Tire Size Calculator
Enter a tire size such as 225/45R17 as width, aspect ratio and rim diameter. Add a second size to see how the change affects diameter and speedometer reading.
Result
- Tire 1 diameter in millimeters
- 634.3 mm
- Tire 1 sidewall height
- 3.99 in
- Tire 1 circumference
- 78.45 in
- Tire 1 revolutions per mile
- 807.6
- Tire 2 diameter in millimeters
- 653.2 mm
- Tire 2 sidewall height
- 3.86 in
- Tire 2 circumference
- 80.79 in
- Tire 2 revolutions per mile
- 784.2
- True speed when the speedometer reads 60
- 61.79
- Ride height change
- 0.37 in
A difference within 3 percent is generally considered acceptable.
Formula
- Sidewall height = width × aspect ratio ÷ 100
- Diameter = rim diameter × 25.4 + 2 × sidewall height (mm)
- Circumference = π × diameter
- Revolutions per mile = 63,360 ÷ circumference in inches
How the Tire Size Calculator works
A metric tire size such as 225/45R17 gives the tread width in millimeters, the sidewall height as a percentage of that width, and the wheel diameter in inches. From those three numbers the overall diameter follows.
The speedometer counts wheel revolutions. Fitting a taller tire means the car travels farther per revolution, so the true speed is higher than the indicated speed by the same percentage as the diameter change.
Worked example
A 225/45R17 has a sidewall of 225 × 0.45 = 101.25 mm. Its diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 in. A 245/40R18 is 653.2 mm, which is 2.98 percent larger, so an indicated 60 is a true 61.79.
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17, 225 is the width in millimeters, 45 is the sidewall height as a percentage of the width, R means radial construction and 17 is the rim diameter in inches.
How much bigger can I go on tire size?
Staying within about 3 percent of the original diameter keeps speedometer error and gearing changes small. Always check physical clearance as well.
Related calculators
- Gear Ratio CalculatorCalculate gear ratio from tooth counts with output speed and torque, or find vehicle speed from engine RPM, axle ratio and tire diameter.
- Power to Weight Ratio CalculatorCalculate power to weight ratio in hp per pound, hp per ton, watts per kilogram and pounds per horsepower for cars, motorcycles and cyclists.
- Engine Displacement CalculatorCalculate engine displacement in cc, liters and cubic inches from bore, stroke and number of cylinders.
- Sleep Cycle CalculatorFind the best time to go to bed or wake up based on 90-minute sleep cycles, so you wake at the end of a cycle and not in the middle of deep sleep.
- Animal Gestation CalculatorCalculate the due date for cows, horses, goats, sheep, pigs, dogs, cats and rabbits from the breeding date, with the normal range.
- ERA CalculatorCalculate earned run average (ERA) and WHIP for baseball and softball pitchers from earned runs, innings pitched, hits and walks.
- Batting Average CalculatorCalculate batting average, on-base percentage, slugging percentage and OPS for baseball and softball from at-bats, hits, walks and extra-base hits.
- Winning Percentage CalculatorCalculate winning percentage from wins, losses and ties, and see how many more wins are needed to reach a target percentage.
- Mouse Sensitivity ConverterConvert mouse sensitivity between Valorant, CS2, Apex Legends, Overwatch 2, Fortnite and other games, with eDPI and cm per 360.
- Nether Portal CalculatorConvert Minecraft Overworld coordinates to Nether coordinates and back, so linked portals line up. One block in the Nether equals eight in the Overworld.
Related unit converters
Calculations run in your browser and nothing you enter is sent to a server. Results are provided for general information. Check critical figures independently before relying on them.