275/40R21
275/40R21 ships as an OEM size on the BMW X5. At 29.66″ diameter with a 4.33″ sidewall (40% of section width), this size turns 680 revolutions per mile.
About 275/40R21
Factory fitments for 275/40R21 include the BMW X5 — meaning replacement tires in this size see steady demand and broad brand availability. At 29.66″ overall diameter, 275/40R21 turns 680 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 265/40R21 drops diameter by 1.06%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 285/40R21 adds 1.06% 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 29.66″ 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 aliases275/40R21
- Diameter
- 29.66″ / 753.4mm
- Section Width
- 10.83″
- Sidewall
- 4.33″ / 110mm
- Circumference
- 93.18″
- Revs/Mile
- 680
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 60.76 mph (97.78 km/h). Your speedometer reads 1.26% low versus actual speed.