305/50R20 vs 285/55R20

Q: Can I replace 305/50R20 with 285/55R20?

285/55R20 is a safe swap for 305/50R20 by the usual ±3% rule: the diameter differs by +1.05%.

285/55R20 is a safe swap for 305/50R20 by the usual ±3% rule: the diameter differs by +1.05%.

305/50R20 32.01" · 285/55R20 32.34" · drawn to scale

Side by side

	305/50R20	285/55R20	Difference
Overall diameter	32.01"	32.34"	+0.33" (+1.05%)
Section width	305 mm	285 mm	-20 mm
Sidewall	6.00"	6.17"	+0.17"
Rim	20"	20"	same
Circumference	100.6"	101.6"	+1.1"
Revs per mile	630	624	-7
Ride height	—	—	+0.17"
Speedometer reads 60 mph	60.0	60.6	+0.6 mph

Speedometer and odometer

With 285/55R20 on a car calibrated for 305/50R20, 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	45.5 mph
60 mph	60.6 mph
70 mph	70.7 mph
80 mph	80.8 mph

Other sizes close to 305/50R20

Questions

Can I replace 305/50R20 with 285/55R20?

285/55R20 is a safe swap for 305/50R20 by the usual ±3% rule: the diameter differs by +1.05%.

What's the difference between 305/50R20 and 285/55R20?

285/55R20 is 0.33" taller and 20 mm narrower; the sidewall is +0.17". At an indicated 60 mph you'd actually be doing 60.6 mph.

Will 285/55R20 fit on 305/50R20 wheels?

Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.