225/40R17 vs 235/35R18

235/35R18 is 0.39 in (9.9 mm) taller than 225/40R17, a +1.62% diameter difference. With the speedometer set for 225/40R17, an indicated 60 mph is 61.0 mph on 235/35R18.

Is this a safe swap?

Diameter difference +1.62%: within the ±3% guideline

By overall diameter alone, 235/35R18 falls inside the ±3% range commonly used as a guideline when replacing 225/40R17. 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.

225/40R17 vs 235/35R18 differences
Measurement	225/40R17	235/35R18	Difference	% diff
Overall diameter	24.09 in (611.8 mm)	24.48 in (621.7 mm)	+0.39 in (+9.9 mm)	+1.62%
Sidewall height	3.54 in (90.0 mm)	3.24 in (82.3 mm)	−0.31 in (−7.7 mm)	−8.61%
Section width	8.86 in (225.0 mm)	9.25 in (235.0 mm)	+0.39 in (+10.0 mm)	+4.44%
Circumference	75.67 in (1,922.0 mm)	76.89 in (1,953.1 mm)	+1.22 in (+31.1 mm)	+1.62%
Revolutions per mile	837	824	−13	−1.59%

True speed with the speedometer calibrated for 225/40R17
Indicated	Actual on 225/40R17	Actual on 235/35R18	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	61.0 mph	+1.0 mph
75 mph	75.0 mph	76.2 mph	+1.2 mph

Ride-height change: 235/35R18 raises the vehicle by 0.19 in (5.0 mm) at the axle (half the +9.9 mm diameter difference).

235/35R18 is 0.39 in taller than 225/40R17 and 10 mm wider, a 1.62% difference in overall diameter.

With the speedometer calibrated for 225/40R17, an indicated 60 mph is actually 61.0 mph on 235/35R18, and revolutions per mile change from 837 to 824.

235/35R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 225/40R17.

Size pages

Other comparisons with 225/40R17

Other comparisons with 235/35R18

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