305/40R20 vs 245/50R20

245/50R20 is 0.04 in (1.0 mm) taller than 305/40R20, a +0.13% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 60.1 mph on 245/50R20.

Is this a safe swap?

Diameter difference +0.13%: under 1%

By overall diameter alone, 245/50R20 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 245/50R20 differences
Measurement	305/40R20	245/50R20	Difference	% diff
Overall diameter	29.61 in (752.0 mm)	29.65 in (753.0 mm)	+0.04 in (+1.0 mm)	+0.13%
Sidewall height	4.80 in (122.0 mm)	4.82 in (122.5 mm)	+0.02 in (+0.5 mm)	+0.41%
Section width	12.01 in (305.0 mm)	9.65 in (245.0 mm)	−2.36 in (−60.0 mm)	−19.67%
Circumference	93.01 in (2,362.5 mm)	93.13 in (2,365.6 mm)	+0.12 in (+3.1 mm)	+0.13%
Revolutions per mile	681	680	−1	−0.13%

True speed with the speedometer calibrated for 305/40R20
Indicated	Actual on 305/40R20	Actual on 245/50R20	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 245/50R20 raises the vehicle by 0.02 in (0.5 mm) at the axle (half the +1.0 mm diameter difference).

245/50R20 is 0.04 in taller than 305/40R20 and 60 mm narrower, a 0.13% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 60.1 mph on 245/50R20, and revolutions per mile change from 681 to 680.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 305/40R20

Other comparisons with 245/50R20

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