305/35R21
305/35R21 is uncommon as OEM fitment but appears as an aftermarket upsize option. At 29.41″ diameter with a 4.20″ sidewall (35% of section width), this size turns 686 revolutions per mile.
About 305/35R21
305/35R21 is less common as a factory fitment but is available as an aftermarket size, so SKU selection is narrower than mainstream OEM sizes. At 29.41″ overall diameter, 305/35R21 turns 686 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 295/35R21 drops diameter by 0.94%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 315/35R21 adds 0.94% 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 29.41″ 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 aliases305/35R21
- Diameter
- 29.41″ / 746.9mm
- Section Width
- 12.01″
- Sidewall
- 4.2″ / 106.75mm
- Circumference
- 92.38″
- Revs/Mile
- 686
285/35R21
- Diameter
- 28.85″ / 732.9mm
- Section Width
- 11.22″
- Sidewall
- 3.93″ / 99.75mm
- Circumference
- 90.65″
- Revs/Mile
- 699
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.88 mph (94.75 km/h). Your speedometer reads 1.87% high versus actual speed.