275/40R18 vs 285/40R18

285/40R18 is 0.31 in (8.0 mm) taller than 275/40R18, a +1.18% diameter difference. With the speedometer set for 275/40R18, an indicated 60 mph is 60.7 mph on 285/40R18.

Is this a safe swap?

Diameter difference +1.18%: within the ±3% guideline

By overall diameter alone, 285/40R18 falls inside the ±3% range commonly used as a guideline when replacing 275/40R18. 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.

275/40R18 vs 285/40R18 differences
Measurement	275/40R18	285/40R18	Difference	% diff
Overall diameter	26.66 in (677.2 mm)	26.98 in (685.2 mm)	+0.31 in (+8.0 mm)	+1.18%
Sidewall height	4.33 in (110.0 mm)	4.49 in (114.0 mm)	+0.16 in (+4.0 mm)	+3.64%
Section width	10.83 in (275.0 mm)	11.22 in (285.0 mm)	+0.39 in (+10.0 mm)	+3.64%
Circumference	83.76 in (2,127.5 mm)	84.75 in (2,152.6 mm)	+0.99 in (+25.1 mm)	+1.18%
Revolutions per mile	756	748	−9	−1.17%

True speed with the speedometer calibrated for 275/40R18
Indicated	Actual on 275/40R18	Actual on 285/40R18	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.9 mph	+0.9 mph

Ride-height change: 285/40R18 raises the vehicle by 0.16 in (4.0 mm) at the axle (half the +8.0 mm diameter difference).

285/40R18 is 0.31 in taller than 275/40R18 and 10 mm wider, a 1.18% difference in overall diameter.

With the speedometer calibrated for 275/40R18, an indicated 60 mph is actually 60.7 mph on 285/40R18, and revolutions per mile change from 756 to 748.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 275/40R18

Other comparisons with 285/40R18

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