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Tire Size Calculator
Enter the three numbers from a tire sidewall — width, aspect ratio and wheel diameter, as in 225/45R17 — to get its sidewall height, overall diameter, circumference and revolutions per mile. Enter a second size to see how much your speedometer will be off after a swap.
- Sidewall height
- 3.99 in
- Circumference
- 78.45 in
- Revolutions per mile
- 807.6
- New tire diameter
- 25.4 in
- New revolutions per mile
- 794
- Diameter difference
- 1.72%
- True speed on the new tires
- 61.03 mph
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the width, aspect ratio and wheel diameter of your current tire.
- Enter the same three numbers for the size you are considering.
- Enter a speedometer reading to see your true speed on the new tires.
- Check the diameter difference.
Worked examples
A 225/45R17 tire, compared with 235/40R18 at an indicated 60 mph
- Sidewall height
- 3.99 in
- Tire diameter
- 24.97 in
- Circumference
- 78.45 in
- Revolutions per mile
- 807.6
- New tire diameter
- 25.4 in
- Diameter difference
- 1.72%
- True speed on the new tires
- 61.03 mph
A truck going from 265/70R17 to 285/70R17 at an indicated 70 mph
- Sidewall height
- 7.3 in
- Tire diameter
- 31.61 in
- Revolutions per mile
- 638.1
- New tire diameter
- 32.71 in
- Diameter difference
- 3.49%
- True speed on the new tires
- 72.44 mph
Reading the sidewall
In 225/45R17, 225 is the tread width in millimeters, 45 is the sidewall height as a percentage of that width, R means radial construction, and 17 is the wheel diameter in inches. Tire sizing mixes metric and imperial units, which is why the formula converts the wheel to millimeters.
The figures are nominal. Actual diameters vary by a few millimeters between brands and shrink slightly as the tread wears.
How big a change is acceptable
A widely used guideline is to keep the overall diameter within about 3% of the original. Beyond that, the speedometer and odometer error grows, gearing and braking feel change, and driver aids such as ABS and stability control may be affected. Larger tires also need checking for clearance at full steering lock and suspension travel.
A larger tire makes the car travel farther per revolution, so the speedometer reads low: you are going faster than it shows.
Questions people ask
What is the diameter of a 225/45R17 tire?
24.97 in (634.3 mm). The sidewall is 101.25 mm, and the tire turns about 808 times per mile.
How much will bigger tires change my speedometer?
By the same percentage as the diameter change. Going from 265/70R17 to 285/70R17 is 3.5% larger, so at an indicated 70 mph you are actually doing about 72.4 mph.
What does the aspect ratio mean on a tire?
It is the sidewall height as a percentage of the tire’s width. A 225/45 tire has a sidewall 45% of 225 mm, which is 101.25 mm.