295/40R20 vs 305/40R20

305/40R20 is 0.31 in (8.0 mm) taller than 295/40R20, a +1.08% diameter difference. With the speedometer set for 295/40R20, an indicated 60 mph is 60.6 mph on 305/40R20.

Is this a safe swap?

Diameter difference +1.08%: within the ±3% guideline

By overall diameter alone, 305/40R20 falls inside the ±3% range commonly used as a guideline when replacing 295/40R20. 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.

295/40R20 vs 305/40R20 differences
Measurement	295/40R20	305/40R20	Difference	% diff
Overall diameter	29.29 in (744.0 mm)	29.61 in (752.0 mm)	+0.31 in (+8.0 mm)	+1.08%
Sidewall height	4.65 in (118.0 mm)	4.80 in (122.0 mm)	+0.16 in (+4.0 mm)	+3.39%
Section width	11.61 in (295.0 mm)	12.01 in (305.0 mm)	+0.39 in (+10.0 mm)	+3.39%
Circumference	92.02 in (2,337.3 mm)	93.01 in (2,362.5 mm)	+0.99 in (+25.1 mm)	+1.08%
Revolutions per mile	689	681	−7	−1.06%

True speed with the speedometer calibrated for 295/40R20
Indicated	Actual on 295/40R20	Actual on 305/40R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.8 mph	+0.8 mph

Ride-height change: 305/40R20 raises the vehicle by 0.16 in (4.0 mm) at the axle (half the +8.0 mm diameter difference).

305/40R20 is 0.31 in taller than 295/40R20 and 10 mm wider, a 1.08% difference in overall diameter.

With the speedometer calibrated for 295/40R20, an indicated 60 mph is actually 60.6 mph on 305/40R20, and revolutions per mile change from 689 to 681.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 295/40R20

Other comparisons with 305/40R20

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