305/30R21 vs 265/40R20

Q: Can I replace 305/30R21 with 265/40R20?

265/40R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.50%.

265/40R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.50%.

305/30R21 28.20" · 265/40R20 28.35" · drawn to scale

Side by side

	305/30R21	265/40R20	Difference
Overall diameter	28.20"	28.35"	+0.14" (+0.50%)
Section width	305 mm	265 mm	-40 mm
Sidewall	3.60"	4.17"	+0.57"
Rim	21"	20"	-1"
Circumference	88.6"	89.1"	+0.4"
Revs per mile	715	711	-4
Ride height	—	—	+0.07"
Speedometer reads 60 mph	60.0	60.3	+0.3 mph

Speedometer and odometer

With 265/40R20 on a car calibrated for 305/30R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.5%.

Speedo shows	Actual speed
30 mph	30.2 mph
45 mph	45.2 mph
60 mph	60.3 mph
70 mph	70.4 mph
80 mph	80.4 mph

Other sizes close to 305/30R21

Questions

Can I replace 305/30R21 with 265/40R20?

265/40R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.50%.

What's the difference between 305/30R21 and 265/40R20?

265/40R20 is 0.14" taller and 40 mm narrower; the sidewall is +0.57". At an indicated 60 mph you'd actually be doing 60.3 mph.

Will 265/40R20 fit on 305/30R21 wheels?

No — 305/30R21 needs a 21" wheel and 265/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.