255/35R18 vs 225/40R18

225/40R18 is 0.06 in (1.5 mm) taller than 255/35R18, a +0.24% diameter difference. With the speedometer set for 255/35R18, an indicated 60 mph is 60.1 mph on 225/40R18.

Is this a safe swap?

Diameter difference +0.24%: under 1%

By overall diameter alone, 225/40R18 is a near match for 255/35R18; 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.

255/35R18 vs 225/40R18 differences
Measurement	255/35R18	225/40R18	Difference	% diff
Overall diameter	25.03 in (635.7 mm)	25.09 in (637.2 mm)	+0.06 in (+1.5 mm)	+0.24%
Sidewall height	3.51 in (89.3 mm)	3.54 in (90.0 mm)	+0.03 in (+0.8 mm)	+0.84%
Section width	10.04 in (255.0 mm)	8.86 in (225.0 mm)	−1.18 in (−30.0 mm)	−11.76%
Circumference	78.63 in (1,997.1 mm)	78.81 in (2,001.8 mm)	+0.19 in (+4.7 mm)	+0.24%
Revolutions per mile	806	804	−2	−0.24%

True speed with the speedometer calibrated for 255/35R18
Indicated	Actual on 255/35R18	Actual on 225/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 225/40R18 raises the vehicle by 0.03 in (0.8 mm) at the axle (half the +1.5 mm diameter difference).

225/40R18 is 0.06 in taller than 255/35R18 and 30 mm narrower, a 0.24% difference in overall diameter.

With the speedometer calibrated for 255/35R18, an indicated 60 mph is actually 60.1 mph on 225/40R18, and revolutions per mile change from 806 to 804.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 255/35R18

Other comparisons with 225/40R18

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