275/65R18 vs 305/50R20

305/50R20 is 0.07 in (1.7 mm) shorter than 275/65R18, a −0.21% diameter difference. With the speedometer set for 275/65R18, an indicated 60 mph is 59.9 mph on 305/50R20.

Is this a safe swap?

Diameter difference −0.21%: under 1%

By overall diameter alone, 305/50R20 is a near match for 275/65R18; 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.

275/65R18 vs 305/50R20 differences
Measurement	275/65R18	305/50R20	Difference	% diff
Overall diameter	32.07 in (814.7 mm)	32.01 in (813.0 mm)	−0.07 in (−1.7 mm)	−0.21%
Sidewall height	7.04 in (178.8 mm)	6.00 in (152.5 mm)	−1.03 in (−26.2 mm)	−14.69%
Section width	10.83 in (275.0 mm)	12.01 in (305.0 mm)	+1.18 in (+30.0 mm)	+10.91%
Circumference	100.77 in (2,559.5 mm)	100.56 in (2,554.1 mm)	−0.21 in (−5.3 mm)	−0.21%
Revolutions per mile	629	630	+1	+0.21%

True speed with the speedometer calibrated for 275/65R18
Indicated	Actual on 275/65R18	Actual on 305/50R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 305/50R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

305/50R20 is 0.07 in shorter than 275/65R18 and 30 mm wider, a 0.21% difference in overall diameter.

With the speedometer calibrated for 275/65R18, an indicated 60 mph is actually 59.9 mph on 305/50R20, and revolutions per mile change from 629 to 630.

305/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/65R18.

Size pages

Other comparisons with 275/65R18

Other comparisons with 305/50R20

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