Tire Size Calculator
Tire size calculator: enter a size such as 225/45R17 to get sidewall height, overall diameter, circumference and revolutions per mile, or compare two sizes. How it works
Use the Tire Size Calculator
About the Tire Size Calculator
A 225/45R17 tire is 634.3 mm (24.97 in) tall, with a 101.3 mm sidewall and 807.6 revolutions per mile. The tire size calculator works that out from the three numbers in the size: sidewall = width × aspect ratio ÷ 100, and diameter = rim × 25.4 + 2 × sidewall.
Dimensions of common tire sizes
| Size | Sidewall height | Overall diameter | Circumference | Revolutions per mile |
|---|---|---|---|---|
| 195/65R15 | 126.8 mm (4.99 in) | 634.5 mm (24.98 in) | 1,993.3 mm (78.48 in) | 807.4 |
| 205/55R16 | 112.8 mm (4.44 in) | 631.9 mm (24.88 in) | 1,985.2 mm (78.16 in) | 810.7 |
| 215/60R16 | 129.0 mm (5.08 in) | 664.4 mm (26.16 in) | 2,087.3 mm (82.18 in) | 771.0 |
| 215/55R17 | 118.3 mm (4.66 in) | 668.3 mm (26.31 in) | 2,099.5 mm (82.66 in) | 766.5 |
| 225/45R17 | 101.3 mm (3.99 in) | 634.3 mm (24.97 in) | 1,992.7 mm (78.45 in) | 807.6 |
| 225/65R17 | 146.3 mm (5.76 in) | 724.3 mm (28.52 in) | 2,275.5 mm (89.58 in) | 707.3 |
| 235/65R17 | 152.8 mm (6.01 in) | 737.3 mm (29.03 in) | 2,316.3 mm (91.19 in) | 694.8 |
| 235/45R18 | 105.8 mm (4.16 in) | 668.7 mm (26.33 in) | 2,100.8 mm (82.71 in) | 766.1 |
| 245/40R18 | 98.0 mm (3.86 in) | 653.2 mm (25.72 in) | 2,052.1 mm (80.79 in) | 784.2 |
| 265/70R17 | 185.5 mm (7.30 in) | 802.8 mm (31.61 in) | 2,522.1 mm (99.29 in) | 638.1 |
| 275/55R20 | 151.3 mm (5.95 in) | 810.5 mm (31.91 in) | 2,546.3 mm (100.25 in) | 632.0 |
| 31x10.50R15 | 203.2 mm (8.00 in) | 787.4 mm (31.00 in) | 2,473.7 mm (97.39 in) | 650.6 |
Nominal dimensions from the numbers in the size. A real tire of the same size differs a little by brand, wear, pressure and load.
True speed after a tire size change
| Original size | New size | Diameter change | True speed at an indicated 60 mph | True speed at an indicated 100 km/h |
|---|---|---|---|---|
| 205/55R16 | 225/45R17 | +2.4 mm (+0.38%) | 60.2 mph | 100.4 km/h |
| 225/45R17 | 235/40R18 | +10.9 mm (+1.72%) | 61.0 mph | 101.7 km/h |
| 225/45R17 | 245/40R18 | +18.9 mm (+2.98%) | 61.8 mph | 103.0 km/h |
| 225/45R17 | 215/45R17 | -9.0 mm (-1.42%) | 59.1 mph | 98.6 km/h |
| 265/70R17 | 285/70R17 | +28.0 mm (+3.49%) | 62.1 mph | 103.5 km/h |
| 265/70R17 | 35x12.50R17 | +86.2 mm (+10.74%) | 66.4 mph | 110.7 km/h |
Assumes the speedometer was exact on the original size. A larger tire travels farther per turn, so the car goes faster than the speedometer says.
Speedometer error from 225/45R17 to 235/40R18
| Speedometer reads | True speed | Difference |
|---|---|---|
| 20 mph | 20.3 mph | +0.3 mph |
| 30 mph | 30.5 mph | +0.5 mph |
| 40 mph | 40.7 mph | +0.7 mph |
| 50 mph | 50.9 mph | +0.9 mph |
| 60 mph | 61.0 mph | +1.0 mph |
| 70 mph | 71.2 mph | +1.2 mph |
| 80 mph | 81.4 mph | +1.4 mph |
| 90 mph | 91.5 mph | +1.5 mph |
The table the calculator shows when 235/40R18 is compared with 225/45R17, with the speedometer calibrated for 225/45R17. The gap grows with speed because the error is a fixed 1.72%.
Frequently asked questions
- What do the numbers on a tire mean?
- In 225/45R17, 225 is the section width in millimeters, 45 is the aspect ratio (the sidewall height as a percentage of the width) and 17 is the rim diameter in inches. R means radial construction. A letter in front, such as P or LT, gives the vehicle class, and figures after the rim, such as 94W, are the load index and speed symbol.
- How tall is a 265/70R17 tire?
- 802.8 mm, or 31.61 in. The sidewall is 265 × 0.70 = 185.5 mm, and the diameter is 17 × 25.4 + 2 × 185.5.
- How much do bigger tires change the speedometer?
- By the same percentage as the circumference. Moving from 265/70R17 to 285/70R17 adds 3.49% of diameter, so an indicated 60 mph is a true 62.1 mph and an indicated 100 km/h is 103.5 km/h. A smaller tire does the opposite: 225/45R17 to 215/45R17 gives a true 59.1 mph. Enter both sizes and the calculator lists the true speed for every 10 mph from 20 to 90.
- What metric size is a 33 inch tire?
- No common metric size is exactly 33 in. 285/70R17 is 32.71 in, 285/75R16 is 32.83 in and 275/70R18 is 33.16 in. A flotation size such as 33x12.50R15 states the 33 in directly.
- Can I enter a flotation size like 31x10.50R15?
- Yes. It is read as 31 in overall diameter and 10.5 in wide on a 15 in rim: 787.4 mm tall with a 203.2 mm sidewall and 650.6 revolutions per mile.
- Will a different tire size fit my car?
- This calculator cannot tell you. It compares nominal dimensions and knows nothing about fender or suspension clearance, wheel width, load index or speed rating. Ask a tire shop or the vehicle maker before you change size.
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