305/70R18 vs 35x13.50R20

35x13.50R20 is 0.19 in (4.8 mm) taller than 305/70R18, a +0.54% diameter difference. With the speedometer set for 305/70R18, an indicated 60 mph is 60.3 mph on 35x13.50R20.

Is this a safe swap?

Diameter difference +0.54%: under 1%

By overall diameter alone, 35x13.50R20 is a near match for 305/70R18; 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.

305/70R18 vs 35x13.50R20 differences
Measurement	305/70R18	35x13.50R20	Difference	% diff
Overall diameter	34.81 in (884.2 mm)	35.00 in (889.0 mm)	+0.19 in (+4.8 mm)	+0.54%
Sidewall height	8.41 in (213.5 mm)	7.50 in (190.5 mm)	−0.91 in (−23.0 mm)	−10.77%
Section width	12.01 in (305.0 mm)	13.50 in (342.9 mm)	+1.49 in (+37.9 mm)	+12.43%
Circumference	109.36 in (2,777.8 mm)	109.96 in (2,792.9 mm)	+0.59 in (+15.1 mm)	+0.54%
Revolutions per mile	579	576	−3	−0.54%

True speed with the speedometer calibrated for 305/70R18
Indicated	Actual on 305/70R18	Actual on 35x13.50R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 35x13.50R20 raises the vehicle by 0.09 in (2.4 mm) at the axle (half the +4.8 mm diameter difference).

35x13.50R20 is 0.19 in taller than 305/70R18 and 38 mm wider, a 0.54% difference in overall diameter.

With the speedometer calibrated for 305/70R18, an indicated 60 mph is actually 60.3 mph on 35x13.50R20, and revolutions per mile change from 579 to 576.

35x13.50R20 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 35x13.50R20

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