275/35R20 vs 285/30R21

Q: Can I replace 275/35R20 with 285/30R21?

285/30R21 is a safe swap for 275/35R20 by the usual ±3% rule: the diameter differs by +0.56%.

285/30R21 is a safe swap for 275/35R20 by the usual ±3% rule: the diameter differs by +0.56%.

275/35R20 27.58" · 285/30R21 27.73" · drawn to scale

Side by side

	275/35R20	285/30R21	Difference
Overall diameter	27.58"	27.73"	+0.15" (+0.56%)
Section width	275 mm	285 mm	+10 mm
Sidewall	3.79"	3.37"	-0.42"
Rim	20"	21"	+1"
Circumference	86.6"	87.1"	+0.5"
Revs per mile	731	727	-4
Ride height	—	—	+0.08"
Speedometer reads 60 mph	60.0	60.3	+0.3 mph

Speedometer and odometer

With 285/30R21 on a car calibrated for 275/35R20, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.6%.

Speedo shows	Actual speed
30 mph	30.2 mph
45 mph	45.3 mph
60 mph	60.3 mph
70 mph	70.4 mph
80 mph	80.4 mph

Other sizes close to 275/35R20

Questions

Can I replace 275/35R20 with 285/30R21?

285/30R21 is a safe swap for 275/35R20 by the usual ±3% rule: the diameter differs by +0.56%.

What's the difference between 275/35R20 and 285/30R21?

285/30R21 is 0.15" taller and 10 mm wider; the sidewall is -0.42". At an indicated 60 mph you'd actually be doing 60.3 mph.

Will 285/30R21 fit on 275/35R20 wheels?

No — 275/35R20 needs a 20" wheel and 285/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.