275/30R21 vs 245/40R20

Q: Can I replace 275/30R21 with 245/40R20?

245/40R20 is a safe swap for 275/30R21 by the usual ±3% rule: the diameter differs by +0.80%.

245/40R20 is a safe swap for 275/30R21 by the usual ±3% rule: the diameter differs by +0.80%.

275/30R21 27.50" · 245/40R20 27.72" · drawn to scale

Side by side

	275/30R21	245/40R20	Difference
Overall diameter	27.50"	27.72"	+0.22" (+0.80%)
Section width	275 mm	245 mm	-30 mm
Sidewall	3.25"	3.86"	+0.61"
Rim	21"	20"	-1"
Circumference	86.4"	87.1"	+0.7"
Revs per mile	733	728	-6
Ride height	—	—	+0.11"
Speedometer reads 60 mph	60.0	60.5	+0.5 mph

Speedometer and odometer

With 245/40R20 on a car calibrated for 275/30R21, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.

Speedo shows	Actual speed
30 mph	30.2 mph
45 mph	45.4 mph
60 mph	60.5 mph
70 mph	70.6 mph
80 mph	80.6 mph

Other sizes close to 275/30R21

Questions

Can I replace 275/30R21 with 245/40R20?

245/40R20 is a safe swap for 275/30R21 by the usual ±3% rule: the diameter differs by +0.80%.

What's the difference between 275/30R21 and 245/40R20?

245/40R20 is 0.22" taller and 30 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 60.5 mph.

Will 245/40R20 fit on 275/30R21 wheels?

No — 275/30R21 needs a 21" wheel and 245/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.