215/55R18 vs 255/35R20

255/35R20 is 0.28 in (7.2 mm) shorter than 215/55R18, a −1.04% diameter difference. With the speedometer set for 215/55R18, an indicated 60 mph is 59.4 mph on 255/35R20.

Is this a safe swap?

Diameter difference −1.04%: within the ±3% guideline

By overall diameter alone, 255/35R20 falls inside the ±3% range commonly used as a guideline when replacing 215/55R18. 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 255/35R20 differences
Measurement	215/55R18	255/35R20	Difference	% diff
Overall diameter	27.31 in (693.7 mm)	27.03 in (686.5 mm)	−0.28 in (−7.2 mm)	−1.04%
Sidewall height	4.66 in (118.3 mm)	3.51 in (89.3 mm)	−1.14 in (−29.0 mm)	−24.52%
Section width	8.46 in (215.0 mm)	10.04 in (255.0 mm)	+1.57 in (+40.0 mm)	+18.60%
Circumference	85.80 in (2,179.3 mm)	84.91 in (2,156.7 mm)	−0.89 in (−22.6 mm)	−1.04%
Revolutions per mile	738	746	+8	+1.05%

True speed with the speedometer calibrated for 215/55R18
Indicated	Actual on 215/55R18	Actual on 255/35R20	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.4 mph	−0.6 mph
75 mph	75.0 mph	74.2 mph	−0.8 mph

Ride-height change: 255/35R20 lowers the vehicle by 0.14 in (3.6 mm) at the axle (half the −7.2 mm diameter difference).

255/35R20 is 0.28 in shorter than 215/55R18 and 40 mm wider, a 1.04% difference in overall diameter.

With the speedometer calibrated for 215/55R18, an indicated 60 mph is actually 59.4 mph on 255/35R20, and revolutions per mile change from 738 to 746.

255/35R20 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 255/35R20

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