305/40R20 vs 305/45R20

305/45R20 is 1.20 in (30.5 mm) taller than 305/40R20, a +4.06% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 62.4 mph on 305/45R20.

Is this a safe swap?

Diameter difference +4.06%: outside the ±3% guideline

By overall diameter alone, 305/45R20 falls outside the ±3% range commonly used as a guideline when replacing 305/40R20; speedometer and ride-height changes are larger. 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 305/45R20 differences
Measurement	305/40R20	305/45R20	Difference	% diff
Overall diameter	29.61 in (752.0 mm)	30.81 in (782.5 mm)	+1.20 in (+30.5 mm)	+4.06%
Sidewall height	4.80 in (122.0 mm)	5.40 in (137.3 mm)	+0.60 in (+15.3 mm)	+12.50%
Section width	12.01 in (305.0 mm)	12.01 in (305.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	93.01 in (2,362.5 mm)	96.78 in (2,458.3 mm)	+3.77 in (+95.8 mm)	+4.06%
Revolutions per mile	681	655	−27	−3.90%

True speed with the speedometer calibrated for 305/40R20
Indicated	Actual on 305/40R20	Actual on 305/45R20	Difference
30 mph	30.0 mph	31.2 mph	+1.2 mph
60 mph	60.0 mph	62.4 mph	+2.4 mph
75 mph	75.0 mph	78.0 mph	+3.0 mph

Ride-height change: 305/45R20 raises the vehicle by 0.60 in (15.3 mm) at the axle (half the +30.5 mm diameter difference).

305/45R20 is 1.20 in taller than 305/40R20 and the same width, a 4.06% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 62.4 mph on 305/45R20, and revolutions per mile change from 681 to 655.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 305/40R20

Other comparisons with 305/45R20

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