225/45R18 vs 255/40R18

255/40R18 is 0.06 in (1.5 mm) taller than 225/45R18, a +0.23% diameter difference. With the speedometer set for 225/45R18, an indicated 60 mph is 60.1 mph on 255/40R18.

Is this a safe swap?

Diameter difference +0.23%: under 1%

By overall diameter alone, 255/40R18 is a near match for 225/45R18; 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.

225/45R18 vs 255/40R18 differences
Measurement	225/45R18	255/40R18	Difference	% diff
Overall diameter	25.97 in (659.7 mm)	26.03 in (661.2 mm)	+0.06 in (+1.5 mm)	+0.23%
Sidewall height	3.99 in (101.3 mm)	4.02 in (102.0 mm)	+0.03 in (+0.8 mm)	+0.74%
Section width	8.86 in (225.0 mm)	10.04 in (255.0 mm)	+1.18 in (+30.0 mm)	+13.33%
Circumference	81.59 in (2,072.5 mm)	81.78 in (2,077.2 mm)	+0.19 in (+4.7 mm)	+0.23%
Revolutions per mile	777	775	−2	−0.23%

True speed with the speedometer calibrated for 225/45R18
Indicated	Actual on 225/45R18	Actual on 255/40R18	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: 255/40R18 raises the vehicle by 0.03 in (0.8 mm) at the axle (half the +1.5 mm diameter difference).

255/40R18 is 0.06 in taller than 225/45R18 and 30 mm wider, a 0.23% difference in overall diameter.

With the speedometer calibrated for 225/45R18, an indicated 60 mph is actually 60.1 mph on 255/40R18, and revolutions per mile change from 777 to 775.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 225/45R18

Other comparisons with 255/40R18

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