315/35R20 vs 245/40R21

Q: Can I replace 315/35R20 with 245/40R21?

245/40R21 is a safe swap for 315/35R20 by the usual ±3% rule: the diameter differs by +0.12%.

245/40R21 is a safe swap for 315/35R20 by the usual ±3% rule: the diameter differs by +0.12%.

315/35R20 28.68" · 245/40R21 28.72" · drawn to scale

Side by side

	315/35R20	245/40R21	Difference
Overall diameter	28.68"	28.72"	+0.04" (+0.12%)
Section width	315 mm	245 mm	-70 mm
Sidewall	4.34"	3.86"	-0.48"
Rim	20"	21"	+1"
Circumference	90.1"	90.2"	+0.1"
Revs per mile	703	702	-1
Ride height	—	—	+0.02"
Speedometer reads 60 mph	60.0	60.1	+0.1 mph

Speedometer and odometer

With 245/40R21 on a car calibrated for 315/35R20, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.1%.

Speedo shows	Actual speed
30 mph	30.0 mph
45 mph	45.1 mph
60 mph	60.1 mph
70 mph	70.1 mph
80 mph	80.1 mph

Other sizes close to 315/35R20

Questions

Can I replace 315/35R20 with 245/40R21?

245/40R21 is a safe swap for 315/35R20 by the usual ±3% rule: the diameter differs by +0.12%.

What's the difference between 315/35R20 and 245/40R21?

245/40R21 is 0.04" taller and 70 mm narrower; the sidewall is -0.48". At an indicated 60 mph you'd actually be doing 60.1 mph.

Will 245/40R21 fit on 315/35R20 wheels?

No — 315/35R20 needs a 20" wheel and 245/40R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.