305/70R18 vs 325/60R20

325/60R20 is 0.54 in (13.8 mm) taller than 305/70R18, a +1.56% diameter difference. With the speedometer set for 305/70R18, an indicated 60 mph is 60.9 mph on 325/60R20.

Is this a safe swap?

Diameter difference +1.56%: within the ±3% guideline

By overall diameter alone, 325/60R20 falls inside the ±3% range commonly used as a guideline when replacing 305/70R18. 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.

305/70R18 vs 325/60R20 differences
Measurement	305/70R18	325/60R20	Difference	% diff
Overall diameter	34.81 in (884.2 mm)	35.35 in (898.0 mm)	+0.54 in (+13.8 mm)	+1.56%
Sidewall height	8.41 in (213.5 mm)	7.68 in (195.0 mm)	−0.73 in (−18.5 mm)	−8.67%
Section width	12.01 in (305.0 mm)	12.80 in (325.0 mm)	+0.79 in (+20.0 mm)	+6.56%
Circumference	109.36 in (2,777.8 mm)	111.07 in (2,821.2 mm)	+1.71 in (+43.4 mm)	+1.56%
Revolutions per mile	579	570	−9	−1.54%

True speed with the speedometer calibrated for 305/70R18
Indicated	Actual on 305/70R18	Actual on 325/60R20	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	60.9 mph	+0.9 mph
75 mph	75.0 mph	76.2 mph	+1.2 mph

Ride-height change: 325/60R20 raises the vehicle by 0.27 in (6.9 mm) at the axle (half the +13.8 mm diameter difference).

325/60R20 is 0.54 in taller than 305/70R18 and 20 mm wider, a 1.56% difference in overall diameter.

With the speedometer calibrated for 305/70R18, an indicated 60 mph is actually 60.9 mph on 325/60R20, and revolutions per mile change from 579 to 570.

325/60R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 305/70R18.

Size pages

Other comparisons with 305/70R18

Other comparisons with 325/60R20

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