315/35R21
315/35R21 is uncommon as OEM fitment but appears as an aftermarket upsize option. At 29.68″ diameter with a 4.34″ sidewall (35% of section width), this size turns 679 revolutions per mile.
About 315/35R21
315/35R21 is less common as a factory fitment but is available as an aftermarket size, so SKU selection is narrower than mainstream OEM sizes. At 29.68″ overall diameter, 315/35R21 turns 679 revolutions per mile, which is the reference point the factory speedometer and odometer were calibrated against. The 35% aspect ratio gives it an ultra-low-profile sidewall tuned for maximum steering precision — at the cost of ride comfort and rim protection. 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 305/35R21 drops diameter by 0.93%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 295/40R21 adds 2.06% 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 29.68″ 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 aliases315/35R21
- Diameter
- 29.68″ / 753.9mm
- Section Width
- 12.4″
- Sidewall
- 4.34″ / 110.25mm
- Circumference
- 93.25″
- Revs/Mile
- 679
295/35R21
- Diameter
- 29.13″ / 739.9mm
- Section Width
- 11.61″
- Sidewall
- 4.06″ / 103.25mm
- Circumference
- 91.51″
- Revs/Mile
- 692
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.89 mph (94.77 km/h). Your speedometer reads 1.86% high versus actual speed.