305/70R18 vs 285/75R18

285/75R18 is 0.02 in (0.5 mm) taller than 305/70R18, a +0.06% diameter difference. With the speedometer set for 305/70R18, an indicated 60 mph is 60.0 mph on 285/75R18.

Is this a safe swap?

Diameter difference +0.06%: under 1%

By overall diameter alone, 285/75R18 is a near match for 305/70R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

305/70R18 vs 285/75R18 differences
Measurement	305/70R18	285/75R18	Difference	% diff
Overall diameter	34.81 in (884.2 mm)	34.83 in (884.7 mm)	+0.02 in (+0.5 mm)	+0.06%
Sidewall height	8.41 in (213.5 mm)	8.42 in (213.8 mm)	+0.01 in (+0.3 mm)	+0.12%
Section width	12.01 in (305.0 mm)	11.22 in (285.0 mm)	−0.79 in (−20.0 mm)	−6.56%
Circumference	109.36 in (2,777.8 mm)	109.42 in (2,779.4 mm)	+0.06 in (+1.6 mm)	+0.06%
Revolutions per mile	579	579	0	−0.06%

True speed with the speedometer calibrated for 305/70R18
Indicated	Actual on 305/70R18	Actual on 285/75R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 285/75R18 raises the vehicle by 0.01 in (0.3 mm) at the axle (half the +0.5 mm diameter difference).

285/75R18 is 0.02 in taller than 305/70R18 and 20 mm narrower, a 0.06% difference in overall diameter.

With the speedometer calibrated for 305/70R18, an indicated 60 mph is actually 60.0 mph on 285/75R18, and revolutions per mile change from 579 to 579.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 305/70R18

Other comparisons with 285/75R18

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.