TireSizesFinder

235/60R18 vs 285/40R20

285/40R20 is 0.13 in (3.2 mm) shorter than 235/60R18, a −0.43% diameter difference. With the speedometer set for 235/60R18, an indicated 60 mph is 59.7 mph on 285/40R20.

Is this a safe swap?

Diameter difference −0.43%: under 1%

By overall diameter alone, 285/40R20 is a near match for 235/60R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 235/60R18 (29.10 in) vs 285/40R20 (28.98 in)235/60R1829.10 in × 9.25 in285/40R2028.98 in × 11.22 in
Side profile and tread width, drawn to the same scale.
235/60R18 vs 285/40R20 differences
Measurement235/60R18285/40R20Difference% diff
Overall diameter29.10 in (739.2 mm)28.98 in (736.0 mm)−0.13 in (−3.2 mm)−0.43%
Sidewall height5.55 in (141.0 mm)4.49 in (114.0 mm)−1.06 in (−27.0 mm)−19.15%
Section width9.25 in (235.0 mm)11.22 in (285.0 mm)+1.97 in (+50.0 mm)+21.28%
Circumference91.43 in (2,322.3 mm)91.03 in (2,312.2 mm)−0.40 in (−10.1 mm)−0.43%
Revolutions per mile693696+3+0.43%
True speed with the speedometer calibrated for 235/60R18
IndicatedActual on 235/60R18Actual on 285/40R20Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.7 mph−0.3 mph
75 mph75.0 mph74.7 mph−0.3 mph

Ride-height change: 285/40R20 lowers the vehicle by 0.06 in (1.6 mm) at the axle (half the −3.2 mm diameter difference).

285/40R20 is 0.13 in shorter than 235/60R18 and 50 mm wider, a 0.43% difference in overall diameter.

With the speedometer calibrated for 235/60R18, an indicated 60 mph is actually 59.7 mph on 285/40R20, and revolutions per mile change from 693 to 696.

285/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/60R18.

Size pages

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Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.