225/50R17 vs 255/40R18

255/40R18 is 0.17 in (4.4 mm) taller than 225/50R17, a +0.67% diameter difference. With the speedometer set for 225/50R17, an indicated 60 mph is 60.4 mph on 255/40R18.

Is this a safe swap?

Diameter difference +0.67%: under 1%

By overall diameter alone, 255/40R18 is a near match for 225/50R17; 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.

225/50R17 vs 255/40R18 differences
Measurement	225/50R17	255/40R18	Difference	% diff
Overall diameter	25.86 in (656.8 mm)	26.03 in (661.2 mm)	+0.17 in (+4.4 mm)	+0.67%
Sidewall height	4.43 in (112.5 mm)	4.02 in (102.0 mm)	−0.41 in (−10.5 mm)	−9.33%
Section width	8.86 in (225.0 mm)	10.04 in (255.0 mm)	+1.18 in (+30.0 mm)	+13.33%
Circumference	81.24 in (2,063.4 mm)	81.78 in (2,077.2 mm)	+0.54 in (+13.8 mm)	+0.67%
Revolutions per mile	780	775	−5	−0.67%

True speed with the speedometer calibrated for 225/50R17
Indicated	Actual on 225/50R17	Actual on 255/40R18	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: 255/40R18 raises the vehicle by 0.09 in (2.2 mm) at the axle (half the +4.4 mm diameter difference).

255/40R18 is 0.17 in taller than 225/50R17 and 30 mm wider, a 0.67% difference in overall diameter.

With the speedometer calibrated for 225/50R17, an indicated 60 mph is actually 60.4 mph on 255/40R18, and revolutions per mile change from 780 to 775.

255/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 225/50R17.

Size pages

Other comparisons with 225/50R17

Other comparisons with 255/40R18

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