215/60R16 vs 255/40R18

255/40R18 is 0.13 in (3.2 mm) shorter than 215/60R16, a −0.48% diameter difference. With the speedometer set for 215/60R16, an indicated 60 mph is 59.7 mph on 255/40R18.

Is this a safe swap?

Diameter difference −0.48%: under 1%

By overall diameter alone, 255/40R18 is a near match for 215/60R16; 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.

215/60R16 vs 255/40R18 differences
Measurement	215/60R16	255/40R18	Difference	% diff
Overall diameter	26.16 in (664.4 mm)	26.03 in (661.2 mm)	−0.13 in (−3.2 mm)	−0.48%
Sidewall height	5.08 in (129.0 mm)	4.02 in (102.0 mm)	−1.06 in (−27.0 mm)	−20.93%
Section width	8.46 in (215.0 mm)	10.04 in (255.0 mm)	+1.57 in (+40.0 mm)	+18.60%
Circumference	82.18 in (2,087.3 mm)	81.78 in (2,077.2 mm)	−0.40 in (−10.1 mm)	−0.48%
Revolutions per mile	771	775	+4	+0.48%

True speed with the speedometer calibrated for 215/60R16
Indicated	Actual on 215/60R16	Actual on 255/40R18	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	74.6 mph	−0.4 mph

Ride-height change: 255/40R18 lowers the vehicle by 0.06 in (1.6 mm) at the axle (half the −3.2 mm diameter difference).

255/40R18 is 0.13 in shorter than 215/60R16 and 40 mm wider, a 0.48% difference in overall diameter.

With the speedometer calibrated for 215/60R16, an indicated 60 mph is actually 59.7 mph on 255/40R18, and revolutions per mile change from 771 to 775.

255/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 215/60R16.

Size pages

Other comparisons with 215/60R16

Other comparisons with 255/40R18

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