TireSizesFinder

275/40R17 vs 235/40R18

235/40R18 is 0.26 in (6.6 mm) shorter than 275/40R17, a −1.01% diameter difference. With the speedometer set for 275/40R17, an indicated 60 mph is 59.4 mph on 235/40R18.

Is this a safe swap?

Diameter difference −1.01%: within the ±3% guideline

By overall diameter alone, 235/40R18 falls inside the ±3% range commonly used as a guideline when replacing 275/40R17. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 275/40R17 (25.66 in) vs 235/40R18 (25.40 in)275/40R1725.66 in × 10.83 in235/40R1825.40 in × 9.25 in
Side profile and tread width, drawn to the same scale.
275/40R17 vs 235/40R18 differences
Measurement275/40R17235/40R18Difference% diff
Overall diameter25.66 in (651.8 mm)25.40 in (645.2 mm)−0.26 in (−6.6 mm)−1.01%
Sidewall height4.33 in (110.0 mm)3.70 in (94.0 mm)−0.63 in (−16.0 mm)−14.55%
Section width10.83 in (275.0 mm)9.25 in (235.0 mm)−1.57 in (−40.0 mm)−14.55%
Circumference80.62 in (2,047.7 mm)79.80 in (2,027.0 mm)−0.82 in (−20.7 mm)−1.01%
Revolutions per mile786794+8+1.02%
True speed with the speedometer calibrated for 275/40R17
IndicatedActual on 275/40R17Actual on 235/40R18Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.4 mph−0.6 mph
75 mph75.0 mph74.2 mph−0.8 mph

Ride-height change: 235/40R18 lowers the vehicle by 0.13 in (3.3 mm) at the axle (half the −6.6 mm diameter difference).

235/40R18 is 0.26 in shorter than 275/40R17 and 40 mm narrower, a 1.01% difference in overall diameter.

With the speedometer calibrated for 275/40R17, an indicated 60 mph is actually 59.4 mph on 235/40R18, and revolutions per mile change from 786 to 794.

235/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 275/40R17.

Size pages

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