TireSizesFinder

215/50R17 vs 235/40R18

235/40R18 is 0.06 in (1.6 mm) shorter than 215/50R17, a −0.25% diameter difference. With the speedometer set for 215/50R17, an indicated 60 mph is 59.9 mph on 235/40R18.

Is this a safe swap?

Diameter difference −0.25%: under 1%

By overall diameter alone, 235/40R18 is a near match for 215/50R17; 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: 215/50R17 (25.46 in) vs 235/40R18 (25.40 in)215/50R1725.46 in × 8.46 in235/40R1825.40 in × 9.25 in
Side profile and tread width, drawn to the same scale.
215/50R17 vs 235/40R18 differences
Measurement215/50R17235/40R18Difference% diff
Overall diameter25.46 in (646.8 mm)25.40 in (645.2 mm)−0.06 in (−1.6 mm)−0.25%
Sidewall height4.23 in (107.5 mm)3.70 in (94.0 mm)−0.53 in (−13.5 mm)−12.56%
Section width8.46 in (215.0 mm)9.25 in (235.0 mm)+0.79 in (+20.0 mm)+9.30%
Circumference80.00 in (2,032.0 mm)79.80 in (2,027.0 mm)−0.20 in (−5.0 mm)−0.25%
Revolutions per mile792794+2+0.25%
True speed with the speedometer calibrated for 215/50R17
IndicatedActual on 215/50R17Actual on 235/40R18Difference
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: 235/40R18 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

235/40R18 is 0.06 in shorter than 215/50R17 and 20 mm wider, a 0.25% difference in overall diameter.

With the speedometer calibrated for 215/50R17, an indicated 60 mph is actually 59.9 mph on 235/40R18, and revolutions per mile change from 792 to 794.

235/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 215/50R17.

Size pages

Other comparisons with 215/50R17

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