245/35R21 vs 285/35R20

Q: Can I replace 245/35R21 with 285/35R20?

285/35R20 is a safe swap for 245/35R21 by the usual ±3% rule: the diameter differs by +0.37%.

285/35R20 is a safe swap for 245/35R21 by the usual ±3% rule: the diameter differs by +0.37%.

245/35R21 27.75" · 285/35R20 27.85" · drawn to scale

Side by side

	245/35R21	285/35R20	Difference
Overall diameter	27.75"	27.85"	+0.10" (+0.37%)
Section width	245 mm	285 mm	+40 mm
Sidewall	3.38"	3.93"	+0.55"
Rim	21"	20"	-1"
Circumference	87.2"	87.5"	+0.3"
Revs per mile	727	724	-3
Ride height	—	—	+0.05"
Speedometer reads 60 mph	60.0	60.2	+0.2 mph

Speedometer and odometer

With 285/35R20 on a car calibrated for 245/35R21, the speedometer reads 59.8 mph when you're really doing 60, and the odometer under-counts by 0.4%.

Speedo shows	Actual speed
30 mph	30.1 mph
45 mph	45.2 mph
60 mph	60.2 mph
70 mph	70.3 mph
80 mph	80.3 mph

Other sizes close to 245/35R21

Questions

Can I replace 245/35R21 with 285/35R20?

285/35R20 is a safe swap for 245/35R21 by the usual ±3% rule: the diameter differs by +0.37%.

What's the difference between 245/35R21 and 285/35R20?

285/35R20 is 0.10" taller and 40 mm wider; the sidewall is +0.55". At an indicated 60 mph you'd actually be doing 60.2 mph.

Will 285/35R20 fit on 245/35R21 wheels?

No — 245/35R21 needs a 21" wheel and 285/35R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.