215/60R17 vs 255/45R18

255/45R18 is 0.12 in (3.1 mm) shorter than 215/60R17, a −0.45% diameter difference. With the speedometer set for 215/60R17, an indicated 60 mph is 59.7 mph on 255/45R18.

Is this a safe swap?

Diameter difference −0.45%: under 1%

By overall diameter alone, 255/45R18 is a near match for 215/60R17; 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.

215/60R17 vs 255/45R18 differences
Measurement	215/60R17	255/45R18	Difference	% diff
Overall diameter	27.16 in (689.8 mm)	27.04 in (686.7 mm)	−0.12 in (−3.1 mm)	−0.45%
Sidewall height	5.08 in (129.0 mm)	4.52 in (114.8 mm)	−0.56 in (−14.2 mm)	−11.05%
Section width	8.46 in (215.0 mm)	10.04 in (255.0 mm)	+1.57 in (+40.0 mm)	+18.60%
Circumference	85.32 in (2,167.1 mm)	84.93 in (2,157.3 mm)	−0.38 in (−9.7 mm)	−0.45%
Revolutions per mile	743	746	+3	+0.45%

True speed with the speedometer calibrated for 215/60R17
Indicated	Actual on 215/60R17	Actual on 255/45R18	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.7 mph	−0.3 mph

Ride-height change: 255/45R18 lowers the vehicle by 0.06 in (1.5 mm) at the axle (half the −3.1 mm diameter difference).

255/45R18 is 0.12 in shorter than 215/60R17 and 40 mm wider, a 0.45% difference in overall diameter.

With the speedometer calibrated for 215/60R17, an indicated 60 mph is actually 59.7 mph on 255/45R18, and revolutions per mile change from 743 to 746.

255/45R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 215/60R17.

Size pages

Other comparisons with 215/60R17

Other comparisons with 255/45R18

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