Decode a tire size into diameter, sidewall, circumference, and revolutions. Request dimensionally similar sizes for another wheel or compare two tires and their speedometer effect.
Examples
265/70R17
The tire is about 31.6 inches (803 mm) tall with a 7.3-inch sidewall and 99.3-inch circumference, making about 638.1 revolutions per mile.
Formula
sidewall mm = width × aspect / 100 and
diameter mm = wheel inches × 25.4 + 2 × sidewall. Circumference is π × diameter.
Compare two sizes and read the speedometer table
Enter the current tire as Tire 1 and the proposed size as Tire 2. Each size needs its width in millimeters, aspect ratio, and wheel diameter in inches. The aspect ratio is a percentage of tire width, not a height in millimeters.
For 265/70R17, the calculated diameter is
17 × 25.4 + 2 × 265 × 0.70 = 802.8 mm. For 265/40R23, it is
23 × 25.4 + 2 × 265 × 0.40 = 796.2 mm. The second diameter is about
0.82% smaller even though the wheel itself is larger, because its sidewall is shorter.
Assuming the speedometer is calibrated exactly for Tire 1, actual speed with
Tire 2 is indicated speed × diameter 2 / diameter 1. At an indicated
60 mph, this gives 60 × 796.2 / 802.8, or about 59.5 mph.
This is a geometric estimate; actual rolling circumference and factory
speedometer calibration can differ.
The alternative-size table is a dimension shortlist, not a fitment approval. Use the Conversion Calculator for ordinary inch/millimeter conversions, and check the vehicle requirements before using any alternative tire or wheel.
Fitment and safety limits
Diameter and speedometer differences are geometric comparisons only. They do not verify wheel width, offset, load index, speed rating, brake or suspension clearance, TPMS behavior, gearing, or legal fitment. NHTSA advises using the original tire size or another size recommended by the vehicle manufacturer; check the door placard, manual, and a qualified installer. See NHTSA TireWise.