235/40R18 vs 275/35R18

275/35R18 is 0.18 in (4.5 mm) taller than 235/40R18, a +0.70% diameter difference. With the speedometer set for 235/40R18, an indicated 60 mph is 60.4 mph on 275/35R18.

Is this a safe swap?

Diameter difference +0.70%: under 1%

By overall diameter alone, 275/35R18 is a near match for 235/40R18; 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.

235/40R18 vs 275/35R18 differences
Measurement	235/40R18	275/35R18	Difference	% diff
Overall diameter	25.40 in (645.2 mm)	25.58 in (649.7 mm)	+0.18 in (+4.5 mm)	+0.70%
Sidewall height	3.70 in (94.0 mm)	3.79 in (96.3 mm)	+0.09 in (+2.3 mm)	+2.39%
Section width	9.25 in (235.0 mm)	10.83 in (275.0 mm)	+1.57 in (+40.0 mm)	+17.02%
Circumference	79.80 in (2,027.0 mm)	80.36 in (2,041.1 mm)	+0.56 in (+14.1 mm)	+0.70%
Revolutions per mile	794	788	−5	−0.69%

True speed with the speedometer calibrated for 235/40R18
Indicated	Actual on 235/40R18	Actual on 275/35R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 275/35R18 raises the vehicle by 0.09 in (2.3 mm) at the axle (half the +4.5 mm diameter difference).

275/35R18 is 0.18 in taller than 235/40R18 and 40 mm wider, a 0.70% difference in overall diameter.

With the speedometer calibrated for 235/40R18, an indicated 60 mph is actually 60.4 mph on 275/35R18, and revolutions per mile change from 794 to 788.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/40R18

Other comparisons with 275/35R18

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