285/75R17 vs 305/70R18

305/70R18 is 0.98 in (24.9 mm) taller than 285/75R17, a +2.90% diameter difference. With the speedometer set for 285/75R17, an indicated 60 mph is 61.7 mph on 305/70R18.

Is this a safe swap?

Diameter difference +2.90%: within the ±3% guideline

By overall diameter alone, 305/70R18 falls inside the ±3% range commonly used as a guideline when replacing 285/75R17. 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.

285/75R17 vs 305/70R18 differences
Measurement	285/75R17	305/70R18	Difference	% diff
Overall diameter	33.83 in (859.3 mm)	34.81 in (884.2 mm)	+0.98 in (+24.9 mm)	+2.90%
Sidewall height	8.42 in (213.8 mm)	8.41 in (213.5 mm)	−0.01 in (−0.3 mm)	−0.12%
Section width	11.22 in (285.0 mm)	12.01 in (305.0 mm)	+0.79 in (+20.0 mm)	+7.02%
Circumference	106.28 in (2,699.6 mm)	109.36 in (2,777.8 mm)	+3.08 in (+78.2 mm)	+2.90%
Revolutions per mile	596	579	−17	−2.82%

True speed with the speedometer calibrated for 285/75R17
Indicated	Actual on 285/75R17	Actual on 305/70R18	Difference
30 mph	30.0 mph	30.9 mph	+0.9 mph
60 mph	60.0 mph	61.7 mph	+1.7 mph
75 mph	75.0 mph	77.2 mph	+2.2 mph

Ride-height change: 305/70R18 raises the vehicle by 0.49 in (12.5 mm) at the axle (half the +24.9 mm diameter difference).

305/70R18 is 0.98 in taller than 285/75R17 and 20 mm wider, a 2.90% difference in overall diameter.

With the speedometer calibrated for 285/75R17, an indicated 60 mph is actually 61.7 mph on 305/70R18, and revolutions per mile change from 596 to 579.

305/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 285/75R17.

Size pages

Other comparisons with 285/75R17

Other comparisons with 305/70R18

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