235/65R18 vs 275/45R20

275/45R20 is 0.28 in (7.2 mm) shorter than 235/65R18, a −0.94% diameter difference. With the speedometer set for 235/65R18, an indicated 60 mph is 59.4 mph on 275/45R20.

Is this a safe swap?

Diameter difference −0.94%: under 1%

By overall diameter alone, 275/45R20 is a near match for 235/65R18; 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.

235/65R18 vs 275/45R20 differences
Measurement	235/65R18	275/45R20	Difference	% diff
Overall diameter	30.03 in (762.7 mm)	29.74 in (755.5 mm)	−0.28 in (−7.2 mm)	−0.94%
Sidewall height	6.01 in (152.8 mm)	4.87 in (123.8 mm)	−1.14 in (−29.0 mm)	−18.99%
Section width	9.25 in (235.0 mm)	10.83 in (275.0 mm)	+1.57 in (+40.0 mm)	+17.02%
Circumference	94.33 in (2,396.1 mm)	93.44 in (2,373.5 mm)	−0.89 in (−22.6 mm)	−0.94%
Revolutions per mile	672	678	+6	+0.95%

True speed with the speedometer calibrated for 235/65R18
Indicated	Actual on 235/65R18	Actual on 275/45R20	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.4 mph	−0.6 mph
75 mph	75.0 mph	74.3 mph	−0.7 mph

Ride-height change: 275/45R20 lowers the vehicle by 0.14 in (3.6 mm) at the axle (half the −7.2 mm diameter difference).

275/45R20 is 0.28 in shorter than 235/65R18 and 40 mm wider, a 0.94% difference in overall diameter.

With the speedometer calibrated for 235/65R18, an indicated 60 mph is actually 59.4 mph on 275/45R20, and revolutions per mile change from 672 to 678.

275/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/65R18.

Size pages

Other comparisons with 235/65R18

Other comparisons with 275/45R20

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