315/45R21
315/45R21 is uncommon as OEM fitment but appears as an aftermarket upsize option. At 32.16″ diameter with a 5.58″ sidewall (45% of section width), this size turns 627 revolutions per mile.
About 315/45R21
315/45R21 is less common as a factory fitment but is available as an aftermarket size, so SKU selection is narrower than mainstream OEM sizes. At 32.16″ overall diameter, 315/45R21 turns 627 revolutions per mile, which is the reference point the factory speedometer and odometer were calibrated against. The 45% aspect ratio gives it a low-profile sidewall that delivers sharp steering response but transmits road imperfections. The 21″ rim pairing is typical for luxury SUV and performance-oriented platforms, so compatible wheels are easy to source new or used. Going one step shorter to 305/45R21 drops diameter by 1.10%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 295/50R21 adds 1.41% diameter — also inside the safe window, with marginally taller ride height. If you are shopping for a direct replacement, any tire within roughly ±0.7″ of the 32.16″ stock diameter (about ±2% in percentage terms) will keep ABS, TPMS, and odometer readings inside the vehicle's designed tolerance.
Compare with another size
275/65R18 or "33" for inch aliases315/45R21
- Diameter
- 32.16″ / 816.9mm
- Section Width
- 12.4″
- Sidewall
- 5.58″ / 141.75mm
- Circumference
- 101.04″
- Revs/Mile
- 627
295/45R21
- Diameter
- 31.45″ / 798.9mm
- Section Width
- 11.61″
- Sidewall
- 5.23″ / 132.75mm
- Circumference
- 98.81″
- Revs/Mile
- 641
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.68 mph (94.43 km/h). Your speedometer reads 2.20% high versus actual speed.