TireSizesFinder

255/50R17 vs 235/50R18

235/50R18 is 0.21 in (5.4 mm) taller than 255/50R17, a +0.79% diameter difference. With the speedometer set for 255/50R17, an indicated 60 mph is 60.5 mph on 235/50R18.

Is this a safe swap?

Diameter difference +0.79%: under 1%

By overall diameter alone, 235/50R18 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 235/50R18 (27.25 in)255/50R1727.04 in × 10.04 in235/50R1827.25 in × 9.25 in
Side profile and tread width, drawn to the same scale.
255/50R17 vs 235/50R18 differences
Measurement255/50R17235/50R18Difference% diff
Overall diameter27.04 in (686.8 mm)27.25 in (692.2 mm)+0.21 in (+5.4 mm)+0.79%
Sidewall height5.02 in (127.5 mm)4.63 in (117.5 mm)−0.39 in (−10.0 mm)−7.84%
Section width10.04 in (255.0 mm)9.25 in (235.0 mm)−0.79 in (−20.0 mm)−7.84%
Circumference84.95 in (2,157.6 mm)85.61 in (2,174.6 mm)+0.67 in (+17.0 mm)+0.79%
Revolutions per mile746740−6−0.78%
True speed with the speedometer calibrated for 255/50R17
IndicatedActual on 255/50R17Actual on 235/50R18Difference
30 mph30.0 mph30.2 mph+0.2 mph
60 mph60.0 mph60.5 mph+0.5 mph
75 mph75.0 mph75.6 mph+0.6 mph

Ride-height change: 235/50R18 raises the vehicle by 0.11 in (2.7 mm) at the axle (half the +5.4 mm diameter difference).

235/50R18 is 0.21 in taller than 255/50R17 and 20 mm narrower, a 0.79% difference in overall diameter.

With the speedometer calibrated for 255/50R17, an indicated 60 mph is actually 60.5 mph on 235/50R18, and revolutions per mile change from 746 to 740.

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

Size pages

Other comparisons with 255/50R17

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