255/45R17 vs 255/40R18

255/40R18 is 0.00 in (0.1 mm) shorter than 255/45R17, a −0.02% diameter difference. With the speedometer set for 255/45R17, an indicated 60 mph is 60.0 mph on 255/40R18.

Is this a safe swap?

Diameter difference −0.02%: under 1%

By overall diameter alone, 255/40R18 is a near match for 255/45R17; 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.

255/45R17 vs 255/40R18 differences
Measurement	255/45R17	255/40R18	Difference	% diff
Overall diameter	26.04 in (661.3 mm)	26.03 in (661.2 mm)	0.00 in (−0.1 mm)	−0.02%
Sidewall height	4.52 in (114.8 mm)	4.02 in (102.0 mm)	−0.50 in (−12.7 mm)	−11.11%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	81.79 in (2,077.5 mm)	81.78 in (2,077.2 mm)	−0.01 in (−0.3 mm)	−0.02%
Revolutions per mile	775	775	0	+0.02%

True speed with the speedometer calibrated for 255/45R17
Indicated	Actual on 255/45R17	Actual on 255/40R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 255/40R18 does not change the vehicle at the axle (half the −0.1 mm diameter difference).

255/40R18 is the same height as 255/45R17 and the same width, a 0.02% difference in overall diameter.

With the speedometer calibrated for 255/45R17, an indicated 60 mph is actually 60.0 mph on 255/40R18, and revolutions per mile change from 775 to 775.

255/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 255/45R17.

Size pages

Other comparisons with 255/45R17

Other comparisons with 255/40R18

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