245/40R21 vs 285/40R20

Q: Can I replace 245/40R21 with 285/40R20?

285/40R20 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +0.90%.

285/40R20 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +0.90%.

245/40R21 28.72" · 285/40R20 28.98" · drawn to scale

Side by side

	245/40R21	285/40R20	Difference
Overall diameter	28.72"	28.98"	+0.26" (+0.90%)
Section width	245 mm	285 mm	+40 mm
Sidewall	3.86"	4.49"	+0.63"
Rim	21"	20"	-1"
Circumference	90.2"	91.0"	+0.8"
Revs per mile	702	696	-6
Ride height	—	—	+0.13"
Speedometer reads 60 mph	60.0	60.5	+0.5 mph

Speedometer and odometer

With 285/40R20 on a car calibrated for 245/40R21, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.9%.

Speedo shows	Actual speed
30 mph	30.3 mph
45 mph	45.4 mph
60 mph	60.5 mph
70 mph	70.6 mph
80 mph	80.7 mph

Other sizes close to 245/40R21

Questions

Can I replace 245/40R21 with 285/40R20?

285/40R20 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +0.90%.

What's the difference between 245/40R21 and 285/40R20?

285/40R20 is 0.26" taller and 40 mm wider; the sidewall is +0.63". At an indicated 60 mph you'd actually be doing 60.5 mph.

Will 285/40R20 fit on 245/40R21 wheels?

No — 245/40R21 needs a 21" wheel and 285/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.