Enter a tire size (e.g. 225/45R17) to calculate sidewall height, overall diameter, circumference and revolutions per distance.
A tire size like 225/45R17 means the tread width is 225mm, the sidewall height is 45% of that width (the aspect ratio), and the wheel diameter is 17 inches. Overall diameter = wheel diameter + 2 x sidewall height. Example: 225/45R17 has a sidewall of 225 x 0.45 = 101.25mm, so the overall diameter is 17 x 25.4 + 2 x 101.25 = 634.3mm, about 25.0 inches.
A tire size like 225/45R17 encodes tread width in mm, sidewall aspect ratio as a percent of that width, and wheel diameter in inches. Multiply width by aspect ratio for sidewall height, then add twice the sidewall to the wheel diameter for overall diameter. Example: 225/45R17 has a sidewall of 101.25mm and an overall diameter of about 634.3mm (24.97 in).
About 634.3 mm (24.97 in) — sidewall height is 225 × 0.45 = 101.25 mm, added twice to the 17 in (431.8 mm) wheel diameter.
The aspect ratio — sidewall height as a percentage of tread width. Here the sidewall is 45% of 225 mm, or 101.25 mm.
101.25 mm — width (225 mm) multiplied by aspect ratio (45 / 100).
Radial — the standard modern tire construction where fabric plies run perpendicular to the direction of travel, as opposed to older diagonal bias-ply tires.
Too slow — a larger tire covers more distance per rotation than the speedometer was calibrated for, so the car is actually going faster than the speedometer shows.
It reads too fast — the car covers less distance per rotation than the original tire, so the actual speed is slower than indicated.
Many tire professionals suggest staying within about 3% of the original overall diameter, to limit speedometer error, clearance issues, and drivetrain strain.
About 645.2 mm (25.40 in) — from this calculator's example table, slightly larger than a 225/45R17.
| Tire size | Sidewall | Overall diameter | Revs per km |
|---|---|---|---|
| 225/45R17 | 101.25 mm | 634.3 mm (24.97 in) | 501.8 |
| 235/40R18 | 94.0 mm | 645.2 mm (25.40 in) | 493.5 |
| 205/55R16 | 112.75 mm | 631.9 mm (24.88 in) | 503.7 |
| 225/45R17 vs 235/40R18 | - | +1.72% diameter | actual 61.0 mph at indicated 60 |
A speedometer measures wheel rotation speed and converts it to a road speed using the diameter of the original tire. A larger-diameter tire covers more distance per rotation, so the car is actually going faster than the speedometer shows; a smaller tire means the car is going slower than indicated.
R stands for radial, the standard tire construction used on almost all passenger vehicles today, where the fabric plies run radially (perpendicular to the direction of travel) rather than diagonally as in older bias-ply tires.
Many tire professionals suggest keeping overall diameter within about 3% of the original size to avoid speedometer error, clearance issues, and strain on the drivetrain, though the acceptable range can depend on the specific vehicle.
No. This calculator only computes geometry from the size code — width, aspect ratio and wheel diameter. It does not account for tread wear (which slightly reduces effective diameter over the tire's life), load index, or speed rating.
This calculator computes tire geometry directly from the standard size code using nominal manufacturer dimensions. Actual measured diameter can vary slightly by brand and tire model, and speedometer error in a real vehicle can also depend on factors the manufacturer calibrated for, so treat results as close estimates rather than exact figures.
Last updated: 2026-08-17