235/45R18 vs 255/30R20

255/30R20 is 0.30 in (7.7 mm) shorter than 235/45R18, a −1.15% diameter difference. With the speedometer set for 235/45R18, an indicated 60 mph is 59.3 mph on 255/30R20.

Is this a safe swap?

Diameter difference −1.15%: within the ±3% guideline

By overall diameter alone, 255/30R20 falls inside the ±3% range commonly used as a guideline when replacing 235/45R18. 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.

235/45R18 vs 255/30R20 differences
Measurement	235/45R18	255/30R20	Difference	% diff
Overall diameter	26.33 in (668.7 mm)	26.02 in (661.0 mm)	−0.30 in (−7.7 mm)	−1.15%
Sidewall height	4.16 in (105.8 mm)	3.01 in (76.5 mm)	−1.15 in (−29.2 mm)	−27.66%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	82.71 in (2,100.8 mm)	81.76 in (2,076.6 mm)	−0.95 in (−24.2 mm)	−1.15%
Revolutions per mile	766	775	+9	+1.16%

True speed with the speedometer calibrated for 235/45R18
Indicated	Actual on 235/45R18	Actual on 255/30R20	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.3 mph	−0.7 mph
75 mph	75.0 mph	74.1 mph	−0.9 mph

Ride-height change: 255/30R20 lowers the vehicle by 0.15 in (3.9 mm) at the axle (half the −7.7 mm diameter difference).

255/30R20 is 0.30 in shorter than 235/45R18 and 20 mm wider, a 1.15% difference in overall diameter.

With the speedometer calibrated for 235/45R18, an indicated 60 mph is actually 59.3 mph on 255/30R20, and revolutions per mile change from 766 to 775.

255/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/45R18.

Size pages

Other comparisons with 235/45R18

Other comparisons with 255/30R20

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