205/50R17 vs 255/35R18

255/35R18 is 0.04 in (1.1 mm) shorter than 205/50R17, a −0.17% diameter difference. With the speedometer set for 205/50R17, an indicated 60 mph is 59.9 mph on 255/35R18.

Is this a safe swap?

Diameter difference −0.17%: under 1%

By overall diameter alone, 255/35R18 is a near match for 205/50R17; 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.

205/50R17 vs 255/35R18 differences
Measurement	205/50R17	255/35R18	Difference	% diff
Overall diameter	25.07 in (636.8 mm)	25.03 in (635.7 mm)	−0.04 in (−1.1 mm)	−0.17%
Sidewall height	4.04 in (102.5 mm)	3.51 in (89.3 mm)	−0.52 in (−13.2 mm)	−12.93%
Section width	8.07 in (205.0 mm)	10.04 in (255.0 mm)	+1.97 in (+50.0 mm)	+24.39%
Circumference	78.76 in (2,000.6 mm)	78.63 in (1,997.1 mm)	−0.14 in (−3.5 mm)	−0.17%
Revolutions per mile	804	806	+1	+0.17%

True speed with the speedometer calibrated for 205/50R17
Indicated	Actual on 205/50R17	Actual on 255/35R18	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.9 mph	−0.1 mph

Ride-height change: 255/35R18 lowers the vehicle by 0.02 in (0.5 mm) at the axle (half the −1.1 mm diameter difference).

255/35R18 is 0.04 in shorter than 205/50R17 and 50 mm wider, a 0.17% difference in overall diameter.

With the speedometer calibrated for 205/50R17, an indicated 60 mph is actually 59.9 mph on 255/35R18, and revolutions per mile change from 804 to 806.

255/35R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 205/50R17.

Size pages

Other comparisons with 205/50R17

Other comparisons with 255/35R18

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