225/45R17 vs 225/40R18

225/40R18 is 0.11 in (2.9 mm) taller than 225/45R17, a +0.46% diameter difference. With the speedometer set for 225/45R17, an indicated 60 mph is 60.3 mph on 225/40R18.

Is this a safe swap?

Diameter difference +0.46%: under 1%

By overall diameter alone, 225/40R18 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 225/40R18 differences
Measurement	225/45R17	225/40R18	Difference	% diff
Overall diameter	24.97 in (634.3 mm)	25.09 in (637.2 mm)	+0.11 in (+2.9 mm)	+0.46%
Sidewall height	3.99 in (101.3 mm)	3.54 in (90.0 mm)	−0.44 in (−11.2 mm)	−11.11%
Section width	8.86 in (225.0 mm)	8.86 in (225.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	78.45 in (1,992.7 mm)	78.81 in (2,001.8 mm)	+0.36 in (+9.1 mm)	+0.46%
Revolutions per mile	808	804	−4	−0.46%

True speed with the speedometer calibrated for 225/45R17
Indicated	Actual on 225/45R17	Actual on 225/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 225/40R18 raises the vehicle by 0.06 in (1.5 mm) at the axle (half the +2.9 mm diameter difference).

225/40R18 is 0.11 in taller than 225/45R17 and the same width, a 0.46% difference in overall diameter.

With the speedometer calibrated for 225/45R17, an indicated 60 mph is actually 60.3 mph on 225/40R18, and revolutions per mile change from 808 to 804.

225/40R18 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 225/40R18

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