TireSizesFinder

235/45R17 vs 225/40R18

225/40R18 is 0.24 in (6.1 mm) shorter than 235/45R17, a −0.95% diameter difference. With the speedometer set for 235/45R17, an indicated 60 mph is 59.4 mph on 225/40R18.

Is this a safe swap?

Diameter difference −0.95%: under 1%

By overall diameter alone, 225/40R18 is a near match for 235/45R17; 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: 235/45R17 (25.33 in) vs 225/40R18 (25.09 in)235/45R1725.33 in × 9.25 in225/40R1825.09 in × 8.86 in
Side profile and tread width, drawn to the same scale.
235/45R17 vs 225/40R18 differences
Measurement235/45R17225/40R18Difference% diff
Overall diameter25.33 in (643.3 mm)25.09 in (637.2 mm)−0.24 in (−6.1 mm)−0.95%
Sidewall height4.16 in (105.8 mm)3.54 in (90.0 mm)−0.62 in (−15.7 mm)−14.89%
Section width9.25 in (235.0 mm)8.86 in (225.0 mm)−0.39 in (−10.0 mm)−4.26%
Circumference79.57 in (2,021.0 mm)78.81 in (2,001.8 mm)−0.75 in (−19.2 mm)−0.95%
Revolutions per mile796804+8+0.96%
True speed with the speedometer calibrated for 235/45R17
IndicatedActual on 235/45R17Actual on 225/40R18Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.4 mph−0.6 mph
75 mph75.0 mph74.3 mph−0.7 mph

Ride-height change: 225/40R18 lowers the vehicle by 0.12 in (3.0 mm) at the axle (half the −6.1 mm diameter difference).

225/40R18 is 0.24 in shorter than 235/45R17 and 10 mm narrower, a 0.95% difference in overall diameter.

With the speedometer calibrated for 235/45R17, an indicated 60 mph is actually 59.4 mph on 225/40R18, and revolutions per mile change from 796 to 804.

225/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/45R17.

Size pages

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