315/40R21
315/40R21 is uncommon as OEM fitment but appears as an aftermarket upsize option. At 30.92″ diameter with a 4.96″ sidewall (40% of section width), this size turns 652 revolutions per mile.
About 315/40R21
315/40R21 is less common as a factory fitment but is available as an aftermarket size, so SKU selection is narrower than mainstream OEM sizes. At 30.92″ overall diameter, 315/40R21 turns 652 revolutions per mile, which is the reference point the factory speedometer and odometer were calibrated against. The 40% 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 305/40R21 drops diameter by 1.02%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 295/45R21 adds 1.72% 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 30.92″ 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 aliases315/40R21
- Diameter
- 30.92″ / 785.4mm
- Section Width
- 12.4″
- Sidewall
- 4.96″ / 126mm
- Circumference
- 97.14″
- Revs/Mile
- 652
295/40R21
- Diameter
- 30.29″ / 769.4mm
- Section Width
- 11.61″
- Sidewall
- 4.65″ / 118mm
- Circumference
- 95.16″
- Revs/Mile
- 666
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.78 mph (94.59 km/h). Your speedometer reads 2.04% high versus actual speed.