305/65R18 vs 285/70R18

Q: Can I replace 305/65R18 with 285/70R18?

285/70R18 is a safe swap for 305/65R18 by the usual ±3% rule: the diameter differs by +0.29%.

285/70R18 is a safe swap for 305/65R18 by the usual ±3% rule: the diameter differs by +0.29%.

305/65R18 33.61" · 285/70R18 33.71" · drawn to scale

Side by side

	305/65R18	285/70R18	Difference
Overall diameter	33.61"	33.71"	+0.10" (+0.29%)
Section width	305 mm	285 mm	-20 mm
Sidewall	7.81"	7.85"	+0.05"
Rim	18"	18"	same
Circumference	105.6"	105.9"	+0.3"
Revs per mile	600	598	-2
Ride height	—	—	+0.05"
Speedometer reads 60 mph	60.0	60.2	+0.2 mph

Speedometer and odometer

With 285/70R18 on a car calibrated for 305/65R18, the speedometer reads 59.8 mph when you're really doing 60, and the odometer under-counts by 0.3%.

Speedo shows	Actual speed
30 mph	30.1 mph
45 mph	45.1 mph
60 mph	60.2 mph
70 mph	70.2 mph
80 mph	80.2 mph

Other sizes close to 305/65R18

Questions

Can I replace 305/65R18 with 285/70R18?

285/70R18 is a safe swap for 305/65R18 by the usual ±3% rule: the diameter differs by +0.29%.

What's the difference between 305/65R18 and 285/70R18?

285/70R18 is 0.10" taller and 20 mm narrower; the sidewall is +0.05". At an indicated 60 mph you'd actually be doing 60.2 mph.

Will 285/70R18 fit on 305/65R18 wheels?

Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.