315/30R21 vs 245/40R21

Q: Can I replace 315/30R21 with 245/40R21?

245/40R21 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.97%.

245/40R21 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.97%.

315/30R21 28.44" · 245/40R21 28.72" · drawn to scale

Side by side

	315/30R21	245/40R21	Difference
Overall diameter	28.44"	28.72"	+0.28" (+0.97%)
Section width	315 mm	245 mm	-70 mm
Sidewall	3.72"	3.86"	+0.14"
Rim	21"	21"	same
Circumference	89.3"	90.2"	+0.9"
Revs per mile	709	702	-7
Ride height	—	—	+0.14"
Speedometer reads 60 mph	60.0	60.6	+0.6 mph

Speedometer and odometer

With 245/40R21 on a car calibrated for 315/30R21, the speedometer reads 59.4 mph when you're really doing 60, and the odometer under-counts by 1.0%.

Speedo shows	Actual speed
30 mph	30.3 mph
45 mph	45.4 mph
60 mph	60.6 mph
70 mph	70.7 mph
80 mph	80.8 mph

Other sizes close to 315/30R21

Questions

Can I replace 315/30R21 with 245/40R21?

245/40R21 is a safe swap for 315/30R21 by the usual ±3% rule: the diameter differs by +0.97%.

What's the difference between 315/30R21 and 245/40R21?

245/40R21 is 0.28" taller and 70 mm narrower; the sidewall is +0.14". At an indicated 60 mph you'd actually be doing 60.6 mph.

Will 245/40R21 fit on 315/30R21 wheels?

Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 245 mm tire and the fender clearance.