275/60R18 vs 305/45R20

305/45R20 is 0.19 in (4.7 mm) shorter than 275/60R18, a −0.60% diameter difference. With the speedometer set for 275/60R18, an indicated 60 mph is 59.6 mph on 305/45R20.

Is this a safe swap?

Diameter difference −0.60%: under 1%

By overall diameter alone, 305/45R20 is a near match for 275/60R18; 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.

275/60R18 vs 305/45R20 differences
Measurement	275/60R18	305/45R20	Difference	% diff
Overall diameter	30.99 in (787.2 mm)	30.81 in (782.5 mm)	−0.19 in (−4.7 mm)	−0.60%
Sidewall height	6.50 in (165.0 mm)	5.40 in (137.3 mm)	−1.09 in (−27.7 mm)	−16.82%
Section width	10.83 in (275.0 mm)	12.01 in (305.0 mm)	+1.18 in (+30.0 mm)	+10.91%
Circumference	97.36 in (2,473.1 mm)	96.78 in (2,458.3 mm)	−0.58 in (−14.8 mm)	−0.60%
Revolutions per mile	651	655	+4	+0.60%

True speed with the speedometer calibrated for 275/60R18
Indicated	Actual on 275/60R18	Actual on 305/45R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.6 mph	−0.4 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 305/45R20 lowers the vehicle by 0.09 in (2.4 mm) at the axle (half the −4.7 mm diameter difference).

305/45R20 is 0.19 in shorter than 275/60R18 and 30 mm wider, a 0.60% difference in overall diameter.

With the speedometer calibrated for 275/60R18, an indicated 60 mph is actually 59.6 mph on 305/45R20, and revolutions per mile change from 651 to 655.

305/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/60R18.

Size pages

Other comparisons with 275/60R18

Other comparisons with 305/45R20

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