225/75R16 vs 255/55R18

255/55R18 is 0.24 in (6.2 mm) shorter than 225/75R16, a −0.83% diameter difference. With the speedometer set for 225/75R16, an indicated 60 mph is 59.5 mph on 255/55R18.

Is this a safe swap?

Diameter difference −0.83%: under 1%

By overall diameter alone, 255/55R18 is a near match for 225/75R16; 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.

225/75R16 vs 255/55R18 differences
Measurement	225/75R16	255/55R18	Difference	% diff
Overall diameter	29.29 in (743.9 mm)	29.04 in (737.7 mm)	−0.24 in (−6.2 mm)	−0.83%
Sidewall height	6.64 in (168.8 mm)	5.52 in (140.3 mm)	−1.12 in (−28.5 mm)	−16.89%
Section width	8.86 in (225.0 mm)	10.04 in (255.0 mm)	+1.18 in (+30.0 mm)	+13.33%
Circumference	92.01 in (2,337.0 mm)	91.24 in (2,317.6 mm)	−0.77 in (−19.5 mm)	−0.83%
Revolutions per mile	689	694	+6	+0.84%

True speed with the speedometer calibrated for 225/75R16
Indicated	Actual on 225/75R16	Actual on 255/55R18	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 255/55R18 lowers the vehicle by 0.12 in (3.1 mm) at the axle (half the −6.2 mm diameter difference).

255/55R18 is 0.24 in shorter than 225/75R16 and 30 mm wider, a 0.83% difference in overall diameter.

With the speedometer calibrated for 225/75R16, an indicated 60 mph is actually 59.5 mph on 255/55R18, and revolutions per mile change from 689 to 694.

255/55R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 225/75R16.

Size pages

Other comparisons with 225/75R16

Other comparisons with 255/55R18

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