305/30R19 vs 235/35R20

235/35R20 is 0.27 in (6.9 mm) taller than 305/30R19, a +1.04% diameter difference. With the speedometer set for 305/30R19, an indicated 60 mph is 60.6 mph on 235/35R20.

Is this a safe swap?

Diameter difference +1.04%: within the ±3% guideline

By overall diameter alone, 235/35R20 falls inside the ±3% range commonly used as a guideline when replacing 305/30R19. 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.

305/30R19 vs 235/35R20 differences
Measurement	305/30R19	235/35R20	Difference	% diff
Overall diameter	26.20 in (665.6 mm)	26.48 in (672.5 mm)	+0.27 in (+6.9 mm)	+1.04%
Sidewall height	3.60 in (91.5 mm)	3.24 in (82.3 mm)	−0.36 in (−9.2 mm)	−10.11%
Section width	12.01 in (305.0 mm)	9.25 in (235.0 mm)	−2.76 in (−70.0 mm)	−22.95%
Circumference	82.32 in (2,091.0 mm)	83.18 in (2,112.7 mm)	+0.85 in (+21.7 mm)	+1.04%
Revolutions per mile	770	762	−8	−1.03%

True speed with the speedometer calibrated for 305/30R19
Indicated	Actual on 305/30R19	Actual on 235/35R20	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: 235/35R20 raises the vehicle by 0.14 in (3.5 mm) at the axle (half the +6.9 mm diameter difference).

235/35R20 is 0.27 in taller than 305/30R19 and 70 mm narrower, a 1.04% difference in overall diameter.

With the speedometer calibrated for 305/30R19, an indicated 60 mph is actually 60.6 mph on 235/35R20, and revolutions per mile change from 770 to 762.

235/35R20 mounts on a 20-inch wheel, a plus-1 fitment from the 19-inch wheel of 305/30R19.

Size pages

Other comparisons with 305/30R19

Other comparisons with 235/35R20

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