275/55R18 vs 235/65R18

235/65R18 is 0.12 in (3.0 mm) taller than 275/55R18, a +0.39% diameter difference. With the speedometer set for 275/55R18, an indicated 60 mph is 60.2 mph on 235/65R18.

Is this a safe swap?

Diameter difference +0.39%: under 1%

By overall diameter alone, 235/65R18 is a near match for 275/55R18; 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.

275/55R18 vs 235/65R18 differences
Measurement	275/55R18	235/65R18	Difference	% diff
Overall diameter	29.91 in (759.7 mm)	30.03 in (762.7 mm)	+0.12 in (+3.0 mm)	+0.39%
Sidewall height	5.95 in (151.3 mm)	6.01 in (152.8 mm)	+0.06 in (+1.5 mm)	+0.99%
Section width	10.83 in (275.0 mm)	9.25 in (235.0 mm)	−1.57 in (−40.0 mm)	−14.55%
Circumference	93.96 in (2,386.7 mm)	94.33 in (2,396.1 mm)	+0.37 in (+9.4 mm)	+0.39%
Revolutions per mile	674	672	−3	−0.39%

True speed with the speedometer calibrated for 275/55R18
Indicated	Actual on 275/55R18	Actual on 235/65R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 235/65R18 raises the vehicle by 0.06 in (1.5 mm) at the axle (half the +3.0 mm diameter difference).

235/65R18 is 0.12 in taller than 275/55R18 and 40 mm narrower, a 0.39% difference in overall diameter.

With the speedometer calibrated for 275/55R18, an indicated 60 mph is actually 60.2 mph on 235/65R18, and revolutions per mile change from 674 to 672.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 275/55R18

Other comparisons with 235/65R18

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