215/65R17 vs 255/50R18

255/50R18 is 0.04 in (0.9 mm) taller than 215/65R17, a +0.13% diameter difference. With the speedometer set for 215/65R17, an indicated 60 mph is 60.1 mph on 255/50R18.

Is this a safe swap?

Diameter difference +0.13%: under 1%

By overall diameter alone, 255/50R18 is a near match for 215/65R17; 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.

215/65R17 vs 255/50R18 differences
Measurement	215/65R17	255/50R18	Difference	% diff
Overall diameter	28.00 in (711.3 mm)	28.04 in (712.2 mm)	+0.04 in (+0.9 mm)	+0.13%
Sidewall height	5.50 in (139.8 mm)	5.02 in (127.5 mm)	−0.48 in (−12.2 mm)	−8.77%
Section width	8.46 in (215.0 mm)	10.04 in (255.0 mm)	+1.57 in (+40.0 mm)	+18.60%
Circumference	87.98 in (2,234.6 mm)	88.09 in (2,237.4 mm)	+0.11 in (+2.8 mm)	+0.13%
Revolutions per mile	720	719	−1	−0.13%

True speed with the speedometer calibrated for 215/65R17
Indicated	Actual on 215/65R17	Actual on 255/50R18	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 255/50R18 raises the vehicle by 0.02 in (0.5 mm) at the axle (half the +0.9 mm diameter difference).

255/50R18 is 0.04 in taller than 215/65R17 and 40 mm wider, a 0.13% difference in overall diameter.

With the speedometer calibrated for 215/65R17, an indicated 60 mph is actually 60.1 mph on 255/50R18, and revolutions per mile change from 720 to 719.

255/50R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 215/65R17.

Size pages

Other comparisons with 215/65R17

Other comparisons with 255/50R18

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