285/40R18 vs 245/35R20

245/35R20 is 0.22 in (5.7 mm) shorter than 285/40R18, a −0.83% diameter difference. With the speedometer set for 285/40R18, an indicated 60 mph is 59.5 mph on 245/35R20.

Is this a safe swap?

Diameter difference −0.83%: under 1%

By overall diameter alone, 245/35R20 is a near match for 285/40R18; 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.

285/40R18 vs 245/35R20 differences
Measurement	285/40R18	245/35R20	Difference	% diff
Overall diameter	26.98 in (685.2 mm)	26.75 in (679.5 mm)	−0.22 in (−5.7 mm)	−0.83%
Sidewall height	4.49 in (114.0 mm)	3.38 in (85.8 mm)	−1.11 in (−28.2 mm)	−24.78%
Section width	11.22 in (285.0 mm)	9.65 in (245.0 mm)	−1.57 in (−40.0 mm)	−14.04%
Circumference	84.75 in (2,152.6 mm)	84.04 in (2,134.7 mm)	−0.71 in (−17.9 mm)	−0.83%
Revolutions per mile	748	754	+6	+0.84%

True speed with the speedometer calibrated for 285/40R18
Indicated	Actual on 285/40R18	Actual on 245/35R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 245/35R20 lowers the vehicle by 0.11 in (2.9 mm) at the axle (half the −5.7 mm diameter difference).

245/35R20 is 0.22 in shorter than 285/40R18 and 40 mm narrower, a 0.83% difference in overall diameter.

With the speedometer calibrated for 285/40R18, an indicated 60 mph is actually 59.5 mph on 245/35R20, and revolutions per mile change from 748 to 754.

245/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/40R18.

Size pages

Other comparisons with 285/40R18

Other comparisons with 245/35R20

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