225/45R17 vs 215/40R18

215/40R18 is 0.20 in (5.1 mm) shorter than 225/45R17, a −0.80% diameter difference. With the speedometer set for 225/45R17, an indicated 60 mph is 59.5 mph on 215/40R18.

Is this a safe swap?

Diameter difference −0.80%: under 1%

By overall diameter alone, 215/40R18 is a near match for 225/45R17; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

225/45R17 vs 215/40R18 differences
Measurement	225/45R17	215/40R18	Difference	% diff
Overall diameter	24.97 in (634.3 mm)	24.77 in (629.2 mm)	−0.20 in (−5.1 mm)	−0.80%
Sidewall height	3.99 in (101.3 mm)	3.39 in (86.0 mm)	−0.60 in (−15.2 mm)	−15.06%
Section width	8.86 in (225.0 mm)	8.46 in (215.0 mm)	−0.39 in (−10.0 mm)	−4.44%
Circumference	78.45 in (1,992.7 mm)	77.82 in (1,976.7 mm)	−0.63 in (−16.0 mm)	−0.80%
Revolutions per mile	808	814	+7	+0.81%

True speed with the speedometer calibrated for 225/45R17
Indicated	Actual on 225/45R17	Actual on 215/40R18	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 215/40R18 lowers the vehicle by 0.10 in (2.5 mm) at the axle (half the −5.1 mm diameter difference).

215/40R18 is 0.20 in shorter than 225/45R17 and 10 mm narrower, a 0.80% difference in overall diameter.

With the speedometer calibrated for 225/45R17, an indicated 60 mph is actually 59.5 mph on 215/40R18, and revolutions per mile change from 808 to 814.

215/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 225/45R17.

Size pages

Other comparisons with 225/45R17

Other comparisons with 215/40R18

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.