215/55R18 vs 225/40R20

225/40R20 is 0.22 in (5.7 mm) shorter than 215/55R18, a −0.82% diameter difference. With the speedometer set for 215/55R18, an indicated 60 mph is 59.5 mph on 225/40R20.

Is this a safe swap?

Diameter difference −0.82%: under 1%

By overall diameter alone, 225/40R20 is a near match for 215/55R18; 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.

215/55R18 vs 225/40R20 differences
Measurement	215/55R18	225/40R20	Difference	% diff
Overall diameter	27.31 in (693.7 mm)	27.09 in (688.0 mm)	−0.22 in (−5.7 mm)	−0.82%
Sidewall height	4.66 in (118.3 mm)	3.54 in (90.0 mm)	−1.11 in (−28.2 mm)	−23.89%
Section width	8.46 in (215.0 mm)	8.86 in (225.0 mm)	+0.39 in (+10.0 mm)	+4.65%
Circumference	85.80 in (2,179.3 mm)	85.10 in (2,161.4 mm)	−0.71 in (−17.9 mm)	−0.82%
Revolutions per mile	738	745	+6	+0.83%

True speed with the speedometer calibrated for 215/55R18
Indicated	Actual on 215/55R18	Actual on 225/40R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 225/40R20 lowers the vehicle by 0.11 in (2.9 mm) at the axle (half the −5.7 mm diameter difference).

225/40R20 is 0.22 in shorter than 215/55R18 and 10 mm wider, a 0.82% difference in overall diameter.

With the speedometer calibrated for 215/55R18, an indicated 60 mph is actually 59.5 mph on 225/40R20, and revolutions per mile change from 738 to 745.

225/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 215/55R18.

Size pages

Other comparisons with 215/55R18

Other comparisons with 225/40R20

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