245/45R18 vs 235/35R20

235/35R20 is 0.20 in (5.2 mm) shorter than 245/45R18, a −0.77% diameter difference. With the speedometer set for 245/45R18, an indicated 60 mph is 59.5 mph on 235/35R20.

Is this a safe swap?

Diameter difference −0.77%: under 1%

By overall diameter alone, 235/35R20 is a near match for 245/45R18; 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.

245/45R18 vs 235/35R20 differences
Measurement	245/45R18	235/35R20	Difference	% diff
Overall diameter	26.68 in (677.7 mm)	26.48 in (672.5 mm)	−0.20 in (−5.2 mm)	−0.77%
Sidewall height	4.34 in (110.3 mm)	3.24 in (82.3 mm)	−1.10 in (−28.0 mm)	−25.40%
Section width	9.65 in (245.0 mm)	9.25 in (235.0 mm)	−0.39 in (−10.0 mm)	−4.08%
Circumference	83.82 in (2,129.1 mm)	83.18 in (2,112.7 mm)	−0.64 in (−16.3 mm)	−0.77%
Revolutions per mile	756	762	+6	+0.77%

True speed with the speedometer calibrated for 245/45R18
Indicated	Actual on 245/45R18	Actual on 235/35R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 235/35R20 lowers the vehicle by 0.10 in (2.6 mm) at the axle (half the −5.2 mm diameter difference).

235/35R20 is 0.20 in shorter than 245/45R18 and 10 mm narrower, a 0.77% difference in overall diameter.

With the speedometer calibrated for 245/45R18, an indicated 60 mph is actually 59.5 mph on 235/35R20, and revolutions per mile change from 756 to 762.

235/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/45R18.

Size pages

Other comparisons with 245/45R18

Other comparisons with 235/35R20

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