275/45R21
275/45R21 ships as an OEM size on the Acura MDX, Ford Explorer, Jeep Grand Cherokee. At 30.74″ diameter with a 4.87″ sidewall (45% of section width), this size turns 656 revolutions per mile.
About 275/45R21
Factory fitments for 275/45R21 include the Acura MDX, Ford Explorer, Jeep Grand Cherokee — meaning replacement tires in this size see steady demand and broad brand availability. At 30.74″ overall diameter, 275/45R21 turns 656 revolutions per mile, which is the reference point the factory speedometer and odometer were calibrated against. The 45% 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 265/45R21 drops diameter by 1.15%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 285/45R21 adds 1.15% 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 30.74″ 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 aliases275/45R21
- Diameter
- 30.74″ / 780.9mm
- Section Width
- 10.83″
- Sidewall
- 4.87″ / 123.75mm
- Circumference
- 96.59″
- Revs/Mile
- 656
255/45R21
- Diameter
- 30.04″ / 762.9mm
- Section Width
- 10.04″
- Sidewall
- 4.52″ / 114.75mm
- Circumference
- 94.36″
- Revs/Mile
- 671
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.62 mph (94.33 km/h). Your speedometer reads 2.31% high versus actual speed.