205/55R17 vs 255/40R18

255/40R18 is 0.15 in (3.9 mm) taller than 205/55R17, a +0.59% diameter difference. With the speedometer set for 205/55R17, an indicated 60 mph is 60.4 mph on 255/40R18.

Is this a safe swap?

Diameter difference +0.59%: under 1%

By overall diameter alone, 255/40R18 is a near match for 205/55R17; 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.

205/55R17 vs 255/40R18 differences
Measurement	205/55R17	255/40R18	Difference	% diff
Overall diameter	25.88 in (657.3 mm)	26.03 in (661.2 mm)	+0.15 in (+3.9 mm)	+0.59%
Sidewall height	4.44 in (112.8 mm)	4.02 in (102.0 mm)	−0.42 in (−10.7 mm)	−9.53%
Section width	8.07 in (205.0 mm)	10.04 in (255.0 mm)	+1.97 in (+50.0 mm)	+24.39%
Circumference	81.30 in (2,065.0 mm)	81.78 in (2,077.2 mm)	+0.48 in (+12.3 mm)	+0.59%
Revolutions per mile	779	775	−5	−0.59%

True speed with the speedometer calibrated for 205/55R17
Indicated	Actual on 205/55R17	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	75.4 mph	+0.4 mph

Ride-height change: 255/40R18 raises the vehicle by 0.08 in (2.0 mm) at the axle (half the +3.9 mm diameter difference).

255/40R18 is 0.15 in taller than 205/55R17 and 50 mm wider, a 0.59% difference in overall diameter.

With the speedometer calibrated for 205/55R17, an indicated 60 mph is actually 60.4 mph on 255/40R18, and revolutions per mile change from 779 to 775.

255/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 205/55R17.

Size pages

Other comparisons with 205/55R17

Other comparisons with 255/40R18

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