315/30R21 vs 275/40R20

Q: Can I replace 315/30R21 with 275/40R20?

275/40R20 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.78%.

275/40R20 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.78%.

315/30R21 28.44" · 275/40R20 28.66" · drawn to scale

Side by side

	315/30R21	275/40R20	Difference
Overall diameter	28.44"	28.66"	+0.22" (+0.78%)
Section width	315 mm	275 mm	-40 mm
Sidewall	3.72"	4.33"	+0.61"
Rim	21"	20"	-1"
Circumference	89.3"	90.0"	+0.7"
Revs per mile	709	704	-5
Ride height	—	—	+0.11"
Speedometer reads 60 mph	60.0	60.5	+0.5 mph

Speedometer and odometer

With 275/40R20 on a car calibrated for 315/30R21, 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	45.3 mph
60 mph	60.5 mph
70 mph	70.5 mph
80 mph	80.6 mph

Other sizes close to 315/30R21

Questions

Can I replace 315/30R21 with 275/40R20?

275/40R20 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.78%.

What's the difference between 315/30R21 and 275/40R20?

275/40R20 is 0.22" taller and 40 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 60.5 mph.

Will 275/40R20 fit on 315/30R21 wheels?

No — 315/30R21 needs a 21" wheel and 275/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.