275/40R21 vs 265/45R21

Q: Can I replace 275/40R21 with 265/45R21?

265/45R21 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +2.46%.

265/45R21 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +2.46%.

275/40R21 29.66" · 265/45R21 30.39" · drawn to scale

Side by side

	275/40R21	265/45R21	Difference
Overall diameter	29.66"	30.39"	+0.73" (+2.46%)
Section width	275 mm	265 mm	-10 mm
Sidewall	4.33"	4.69"	+0.36"
Rim	21"	21"	same
Circumference	93.2"	95.5"	+2.3"
Revs per mile	680	664	-16
Ride height	—	—	+0.36"
Speedometer reads 60 mph	60.0	61.5	+1.5 mph

Speedometer and odometer

With 265/45R21 on a car calibrated for 275/40R21, the speedometer reads 58.6 mph when you're really doing 60, and the odometer under-counts by 2.5%.

Speedo shows	Actual speed
30 mph	30.7 mph
45 mph	46.1 mph
60 mph	61.5 mph
70 mph	71.7 mph
80 mph	82.0 mph

Other sizes close to 275/40R21

Questions

Can I replace 275/40R21 with 265/45R21?

265/45R21 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +2.46%.

What's the difference between 275/40R21 and 265/45R21?

265/45R21 is 0.73" taller and 10 mm narrower; the sidewall is +0.36". At an indicated 60 mph you'd actually be doing 61.5 mph.

Will 265/45R21 fit on 275/40R21 wheels?

Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 265 mm tire and the fender clearance.