255/65R18 vs 305/45R20

305/45R20 is 0.24 in (6.2 mm) shorter than 255/65R18, a −0.79% diameter difference. With the speedometer set for 255/65R18, an indicated 60 mph is 59.5 mph on 305/45R20.

Is this a safe swap?

Diameter difference −0.79%: under 1%

By overall diameter alone, 305/45R20 is a near match for 255/65R18; 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.

255/65R18 vs 305/45R20 differences
Measurement	255/65R18	305/45R20	Difference	% diff
Overall diameter	31.05 in (788.7 mm)	30.81 in (782.5 mm)	−0.24 in (−6.2 mm)	−0.79%
Sidewall height	6.53 in (165.8 mm)	5.40 in (137.3 mm)	−1.12 in (−28.5 mm)	−17.19%
Section width	10.04 in (255.0 mm)	12.01 in (305.0 mm)	+1.97 in (+50.0 mm)	+19.61%
Circumference	97.55 in (2,477.8 mm)	96.78 in (2,458.3 mm)	−0.77 in (−19.5 mm)	−0.79%
Revolutions per mile	650	655	+5	+0.79%

True speed with the speedometer calibrated for 255/65R18
Indicated	Actual on 255/65R18	Actual on 305/45R20	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: 305/45R20 lowers the vehicle by 0.12 in (3.1 mm) at the axle (half the −6.2 mm diameter difference).

305/45R20 is 0.24 in shorter than 255/65R18 and 50 mm wider, a 0.79% difference in overall diameter.

With the speedometer calibrated for 255/65R18, an indicated 60 mph is actually 59.5 mph on 305/45R20, and revolutions per mile change from 650 to 655.

305/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/65R18.

Size pages

Other comparisons with 255/65R18

Other comparisons with 305/45R20

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