285/70R18 vs 305/70R18

305/70R18 is 1.10 in (28.0 mm) taller than 285/70R18, a +3.27% diameter difference. With the speedometer set for 285/70R18, an indicated 60 mph is 62.0 mph on 305/70R18.

Is this a safe swap?

Diameter difference +3.27%: outside the ±3% guideline

By overall diameter alone, 305/70R18 falls outside the ±3% range commonly used as a guideline when replacing 285/70R18; speedometer and ride-height changes are larger. 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.

285/70R18 vs 305/70R18 differences
Measurement	285/70R18	305/70R18	Difference	% diff
Overall diameter	33.71 in (856.2 mm)	34.81 in (884.2 mm)	+1.10 in (+28.0 mm)	+3.27%
Sidewall height	7.85 in (199.5 mm)	8.41 in (213.5 mm)	+0.55 in (+14.0 mm)	+7.02%
Section width	11.22 in (285.0 mm)	12.01 in (305.0 mm)	+0.79 in (+20.0 mm)	+7.02%
Circumference	105.90 in (2,689.8 mm)	109.36 in (2,777.8 mm)	+3.46 in (+88.0 mm)	+3.27%
Revolutions per mile	598	579	−19	−3.17%

True speed with the speedometer calibrated for 285/70R18
Indicated	Actual on 285/70R18	Actual on 305/70R18	Difference
30 mph	30.0 mph	31.0 mph	+1.0 mph
60 mph	60.0 mph	62.0 mph	+2.0 mph
75 mph	75.0 mph	77.5 mph	+2.5 mph

Ride-height change: 305/70R18 raises the vehicle by 0.55 in (14.0 mm) at the axle (half the +28.0 mm diameter difference).

305/70R18 is 1.10 in taller than 285/70R18 and 20 mm wider, a 3.27% difference in overall diameter.

With the speedometer calibrated for 285/70R18, an indicated 60 mph is actually 62.0 mph on 305/70R18, and revolutions per mile change from 598 to 579.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 285/70R18

Other comparisons with 305/70R18

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