305/40R21
305/40R21 is uncommon as OEM fitment but appears as an aftermarket upsize option. At 30.61″ diameter with a 4.80″ sidewall (40% of section width), this size turns 659 revolutions per mile.
About 305/40R21
305/40R21 is less common as a factory fitment but is available as an aftermarket size, so SKU selection is narrower than mainstream OEM sizes. At 30.61″ overall diameter, 305/40R21 turns 659 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 295/40R21 drops diameter by 1.03%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 285/45R21 adds 1.61% 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 30.61″ 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 aliases305/40R21
- Diameter
- 30.61″ / 777.4mm
- Section Width
- 12.01″
- Sidewall
- 4.8″ / 122mm
- Circumference
- 96.15″
- Revs/Mile
- 659
285/40R21
- Diameter
- 29.98″ / 761.4mm
- Section Width
- 11.22″
- Sidewall
- 4.49″ / 114mm
- Circumference
- 94.17″
- Revs/Mile
- 673
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.77 mph (94.57 km/h). Your speedometer reads 2.06% high versus actual speed.