305/40R20 vs 275/45R20

275/45R20 is 0.14 in (3.5 mm) taller than 305/40R20, a +0.47% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 60.3 mph on 275/45R20.

Is this a safe swap?

Diameter difference +0.47%: under 1%

By overall diameter alone, 275/45R20 is a near match for 305/40R20; 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.

305/40R20 vs 275/45R20 differences
Measurement	305/40R20	275/45R20	Difference	% diff
Overall diameter	29.61 in (752.0 mm)	29.74 in (755.5 mm)	+0.14 in (+3.5 mm)	+0.47%
Sidewall height	4.80 in (122.0 mm)	4.87 in (123.8 mm)	+0.07 in (+1.8 mm)	+1.43%
Section width	12.01 in (305.0 mm)	10.83 in (275.0 mm)	−1.18 in (−30.0 mm)	−9.84%
Circumference	93.01 in (2,362.5 mm)	93.44 in (2,373.5 mm)	+0.43 in (+11.0 mm)	+0.47%
Revolutions per mile	681	678	−3	−0.46%

True speed with the speedometer calibrated for 305/40R20
Indicated	Actual on 305/40R20	Actual on 275/45R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 275/45R20 raises the vehicle by 0.07 in (1.8 mm) at the axle (half the +3.5 mm diameter difference).

275/45R20 is 0.14 in taller than 305/40R20 and 30 mm narrower, a 0.47% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 60.3 mph on 275/45R20, and revolutions per mile change from 681 to 678.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 305/40R20

Other comparisons with 275/45R20

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