255/40R18 vs 255/30R20

255/30R20 is 0.01 in (0.2 mm) shorter than 255/40R18, a −0.03% diameter difference. With the speedometer set for 255/40R18, an indicated 60 mph is 60.0 mph on 255/30R20.

Is this a safe swap?

Diameter difference −0.03%: under 1%

By overall diameter alone, 255/30R20 is a near match for 255/40R18; 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.

255/40R18 vs 255/30R20 differences
Measurement	255/40R18	255/30R20	Difference	% diff
Overall diameter	26.03 in (661.2 mm)	26.02 in (661.0 mm)	−0.01 in (−0.2 mm)	−0.03%
Sidewall height	4.02 in (102.0 mm)	3.01 in (76.5 mm)	−1.00 in (−25.5 mm)	−25.00%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	81.78 in (2,077.2 mm)	81.76 in (2,076.6 mm)	−0.02 in (−0.6 mm)	−0.03%
Revolutions per mile	775	775	0	+0.03%

True speed with the speedometer calibrated for 255/40R18
Indicated	Actual on 255/40R18	Actual on 255/30R20
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: 255/30R20 lowers the vehicle by 0.00 in (0.1 mm) at the axle (half the −0.2 mm diameter difference).

255/30R20 is 0.01 in shorter than 255/40R18 and the same width, a 0.03% difference in overall diameter.

With the speedometer calibrated for 255/40R18, an indicated 60 mph is actually 60.0 mph on 255/30R20, and revolutions per mile change from 775 to 775.

255/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/40R18.

Size pages

Other comparisons with 255/40R18

Other comparisons with 255/30R20

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