Tire Size Calculator
Enter the size printed on your tire, such as 225/45R17, and optionally a replacement size. The calculator shows the overall diameter, width, sidewall height, circumference and how your speedometer reading would change.
Last updated October 23, 2026
Printed on the sidewall, e.g. 225/45R17
Leave blank to see one tire only
Result
24.97 in diameter
| Measure | Value |
|---|---|
| Overall diameter | 24.97 in |
| Section width | 8.86 in |
| Sidewall height | 3.99 in |
| Circumference | 78.5 in |
| Revolutions per mile | 808 |
| Replacement diameter | 25.72 in |
| Diameter change | +2.98% |
| Speedometer at 60 mph | 61.8 mph actual |
Dimensions are calculated from the size code. Real tires vary slightly by brand and wear.
Show the work
Formula: diameter = rim + 2 x (width x aspect% / 25.4)
- Current 225/45R17: sidewall = 225 x 45% = 101.25 mm = 3.99 in (mm / 25.4).
- Diameter = 17 + 2 x 3.99 = 24.97 in. Circumference = pi x diameter = 78.5 in.
- Revolutions per mile = 63,360 in / 78.5 in = 808.
- Replacement 245/40R18: sidewall = 245 x 40% = 98 mm = 3.86 in (mm / 25.4).
- Diameter = 18 + 2 x 3.86 = 25.72 in. Circumference = pi x diameter = 80.8 in.
- Revolutions per mile = 63,360 in / 80.8 in = 784.
- Diameter change = (25.72 / 24.97 - 1) x 100 = +2.98%.
- Speedometer: 60 mph x 25.72 / 24.97 = 61.8 mph actual.
Result: 24.97 in diameter
How tire dimensions are calculated
sidewall = width × aspect ÷ 100
diameter = rim + 2 × sidewall
circumference = π × diameter
A metric tire size such as 225/45R17 means a 225 mm wide tire, a sidewall 45% as tall as the width, and a 17-inch rim. The sidewall is 225 × 0.45 = 101.25 mm (3.99 in), so the overall diameter is 17 + 2 × 3.99 = 24.97 inches and the circumference is π × 24.97 = 78.4 inches.
The speedometer counts wheel turns, so a taller tire covers more road per turn. The true speed equals the shown speed multiplied by the new diameter divided by the old diameter. Going from 225/45R17 to 245/40R18 makes the tire 2.98% taller, so a speedometer showing 60 mph means about 61.8 mph. A difference of more than about 3% can affect the speedometer, gearing and clearance, so check your vehicle’s placard or manual before changing size.
See how we calculate for the formulas, sources and review schedule behind every calculator.
How to use the Tire Size Calculator
- Find the size on the tire sidewall, for example 225/45R17.
- Enter it as your current size, and optionally enter a replacement size.
- Read the diameter, sidewall, circumference and speedometer difference, then use Copy, Print or Share.
Worked example
Replacing 225/45R17 (24.97 in tall) with 245/40R18 (25.71 in tall) is 2.98% taller. At a speedometer reading of 60 mph you would really travel about 61.8 mph.
| Size | Diameter (in) | Sidewall (in) | Revs per mile |
|---|---|---|---|
| 205/55R16 | 24.9 | 4.44 | 811 |
| 215/60R16 | 26.2 | 5.08 | 771 |
| 225/45R17 | 25.0 | 3.99 | 808 |
| 235/45R18 | 26.3 | 4.16 | 766 |
| 245/40R18 | 25.7 | 3.86 | 784 |
Frequently asked questions
What do the numbers in 225/45R17 mean?
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 wheel diameter in inches.
How much tire diameter change is safe?
A common guideline is to stay within about 3% of the original diameter. Check clearance, load index and speed rating too.
Does a bigger tire make my speedometer read low?
Yes. A taller tire travels farther per revolution, so you are going faster than the speedometer shows.
Can I enter a size with a load rating?
Enter only the width, aspect ratio and rim, for example 225/45R17. A leading P or LT and a ZR are accepted; load index and speed rating are ignored.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser and nothing you enter is saved or uploaded.