305/60R18 vs 285/70R17

Q: Can I replace 305/60R18 with 285/70R17?

285/70R17 is a safe swap for 305/60R18 by the usual ±3% rule: the diameter differs by +0.92%.

285/70R17 is a safe swap for 305/60R18 by the usual ±3% rule: the diameter differs by +0.92%.

305/60R18 32.41" · 285/70R17 32.71" · drawn to scale

Side by side

	305/60R18	285/70R17	Difference
Overall diameter	32.41"	32.71"	+0.30" (+0.92%)
Section width	305 mm	285 mm	-20 mm
Sidewall	7.20"	7.85"	+0.65"
Rim	18"	17"	-1"
Circumference	101.8"	102.8"	+0.9"
Revs per mile	622	617	-6
Ride height	—	—	+0.15"
Speedometer reads 60 mph	60.0	60.6	+0.6 mph

Speedometer and odometer

With 285/70R17 on a car calibrated for 305/60R18, 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	60.6 mph
70 mph	70.6 mph
80 mph	80.7 mph

Other sizes close to 305/60R18

Questions

Can I replace 305/60R18 with 285/70R17?

285/70R17 is a safe swap for 305/60R18 by the usual ±3% rule: the diameter differs by +0.92%.

What's the difference between 305/60R18 and 285/70R17?

285/70R17 is 0.30" taller and 20 mm narrower; the sidewall is +0.65". At an indicated 60 mph you'd actually be doing 60.6 mph.

Will 285/70R17 fit on 305/60R18 wheels?

No — 305/60R18 needs a 18" wheel and 285/70R17 a 17" wheel. This is a plus/minus-size swap that changes wheels too.