285/40R21
285/40R21 ships as an OEM size on the Audi Q7, Porsche Cayenne. At 29.98″ diameter with a 4.49″ sidewall (40% of section width), this size turns 673 revolutions per mile.
About 285/40R21
Factory fitments for 285/40R21 include the Audi Q7, Porsche Cayenne — meaning replacement tires in this size see steady demand and broad brand availability. At 29.98″ overall diameter, 285/40R21 turns 673 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 275/40R21 drops diameter by 1.05%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 265/45R21 adds 1.38% 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 29.98″ 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 aliases285/40R21
- Diameter
- 29.98″ / 761.4mm
- Section Width
- 11.22″
- Sidewall
- 4.49″ / 114mm
- Circumference
- 94.17″
- Revs/Mile
- 673
265/40R21
- Diameter
- 29.35″ / 745.4mm
- Section Width
- 10.43″
- Sidewall
- 4.17″ / 106mm
- Circumference
- 92.19″
- Revs/Mile
- 687
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.74 mph (94.53 km/h). Your speedometer reads 2.10% high versus actual speed.