TireSizesFinder

235/50R17 vs 255/40R18

255/40R18 is 0.22 in (5.6 mm) shorter than 235/50R17, a −0.84% diameter difference. With the speedometer set for 235/50R17, an indicated 60 mph is 59.5 mph on 255/40R18.

Is this a safe swap?

Diameter difference −0.84%: under 1%

By overall diameter alone, 255/40R18 is a near match for 235/50R17; 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: 235/50R17 (26.25 in) vs 255/40R18 (26.03 in)235/50R1726.25 in × 9.25 in255/40R1826.03 in × 10.04 in
Side profile and tread width, drawn to the same scale.
235/50R17 vs 255/40R18 differences
Measurement235/50R17255/40R18Difference% diff
Overall diameter26.25 in (666.8 mm)26.03 in (661.2 mm)−0.22 in (−5.6 mm)−0.84%
Sidewall height4.63 in (117.5 mm)4.02 in (102.0 mm)−0.61 in (−15.5 mm)−13.19%
Section width9.25 in (235.0 mm)10.04 in (255.0 mm)+0.79 in (+20.0 mm)+8.51%
Circumference82.47 in (2,094.8 mm)81.78 in (2,077.2 mm)−0.69 in (−17.6 mm)−0.84%
Revolutions per mile768775+7+0.85%
True speed with the speedometer calibrated for 235/50R17
IndicatedActual on 235/50R17Actual on 255/40R18Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.5 mph−0.5 mph
75 mph75.0 mph74.4 mph−0.6 mph

Ride-height change: 255/40R18 lowers the vehicle by 0.11 in (2.8 mm) at the axle (half the −5.6 mm diameter difference).

255/40R18 is 0.22 in shorter than 235/50R17 and 20 mm wider, a 0.84% difference in overall diameter.

With the speedometer calibrated for 235/50R17, an indicated 60 mph is actually 59.5 mph on 255/40R18, and revolutions per mile change from 768 to 775.

255/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/50R17.

Size pages

Other comparisons with 235/50R17

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