305/40R22 vs 275/50R21

Q: Can I replace 305/40R22 with 275/50R21?

275/50R21 is a safe swap for 305/40R22 by the usual ±3% rule: the diameter differs by +0.70%.

275/50R21 is a safe swap for 305/40R22 by the usual ±3% rule: the diameter differs by +0.70%.

305/40R22 31.61" · 275/50R21 31.83" · drawn to scale

Side by side

	305/40R22	275/50R21	Difference
Overall diameter	31.61"	31.83"	+0.22" (+0.70%)
Section width	305 mm	275 mm	-30 mm
Sidewall	4.80"	5.41"	+0.61"
Rim	22"	21"	-1"
Circumference	99.3"	100.0"	+0.7"
Revs per mile	638	634	-4
Ride height	—	—	+0.11"
Speedometer reads 60 mph	60.0	60.4	+0.4 mph

Speedometer and odometer

With 275/50R21 on a car calibrated for 305/40R22, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.7%.

Speedo shows	Actual speed
30 mph	30.2 mph
45 mph	45.3 mph
60 mph	60.4 mph
70 mph	70.5 mph
80 mph	80.6 mph

Other sizes close to 305/40R22

Questions

Can I replace 305/40R22 with 275/50R21?

275/50R21 is a safe swap for 305/40R22 by the usual ±3% rule: the diameter differs by +0.70%.

What's the difference between 305/40R22 and 275/50R21?

275/50R21 is 0.22" taller and 30 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 60.4 mph.

Will 275/50R21 fit on 305/40R22 wheels?

No — 305/40R22 needs a 22" wheel and 275/50R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.