TireSizesFinder

255/50R17 vs 285/40R18

285/40R18 is 0.06 in (1.6 mm) shorter than 255/50R17, a −0.23% diameter difference. With the speedometer set for 255/50R17, an indicated 60 mph is 59.9 mph on 285/40R18.

Is this a safe swap?

Diameter difference −0.23%: under 1%

By overall diameter alone, 285/40R18 is a near match for 255/50R17; 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: 255/50R17 (27.04 in) vs 285/40R18 (26.98 in)255/50R1727.04 in × 10.04 in285/40R1826.98 in × 11.22 in
Side profile and tread width, drawn to the same scale.
255/50R17 vs 285/40R18 differences
Measurement255/50R17285/40R18Difference% diff
Overall diameter27.04 in (686.8 mm)26.98 in (685.2 mm)−0.06 in (−1.6 mm)−0.23%
Sidewall height5.02 in (127.5 mm)4.49 in (114.0 mm)−0.53 in (−13.5 mm)−10.59%
Section width10.04 in (255.0 mm)11.22 in (285.0 mm)+1.18 in (+30.0 mm)+11.76%
Circumference84.95 in (2,157.6 mm)84.75 in (2,152.6 mm)−0.20 in (−5.0 mm)−0.23%
Revolutions per mile746748+2+0.23%
True speed with the speedometer calibrated for 255/50R17
IndicatedActual on 255/50R17Actual on 285/40R18Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.9 mph−0.1 mph
75 mph75.0 mph74.8 mph−0.2 mph

Ride-height change: 285/40R18 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

285/40R18 is 0.06 in shorter than 255/50R17 and 30 mm wider, a 0.23% difference in overall diameter.

With the speedometer calibrated for 255/50R17, an indicated 60 mph is actually 59.9 mph on 285/40R18, and revolutions per mile change from 746 to 748.

285/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 255/50R17.

Size pages

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