215/45R17 vs 215/40R18

215/40R18 is 0.15 in (3.9 mm) taller than 215/45R17, a +0.62% diameter difference. With the speedometer set for 215/45R17, an indicated 60 mph is 60.4 mph on 215/40R18.

Is this a safe swap?

Diameter difference +0.62%: under 1%

By overall diameter alone, 215/40R18 is a near match for 215/45R17; 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.

215/45R17 vs 215/40R18 differences
Measurement	215/45R17	215/40R18	Difference	% diff
Overall diameter	24.62 in (625.3 mm)	24.77 in (629.2 mm)	+0.15 in (+3.9 mm)	+0.62%
Sidewall height	3.81 in (96.8 mm)	3.39 in (86.0 mm)	−0.42 in (−10.7 mm)	−11.11%
Section width	8.46 in (215.0 mm)	8.46 in (215.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	77.34 in (1,964.4 mm)	77.82 in (1,976.7 mm)	+0.48 in (+12.3 mm)	+0.62%
Revolutions per mile	819	814	−5	−0.62%

True speed with the speedometer calibrated for 215/45R17
Indicated	Actual on 215/45R17	Actual on 215/40R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 215/40R18 raises the vehicle by 0.08 in (2.0 mm) at the axle (half the +3.9 mm diameter difference).

215/40R18 is 0.15 in taller than 215/45R17 and the same width, a 0.62% difference in overall diameter.

With the speedometer calibrated for 215/45R17, an indicated 60 mph is actually 60.4 mph on 215/40R18, and revolutions per mile change from 819 to 814.

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

Size pages

Other comparisons with 215/45R17

Other comparisons with 215/40R18

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