235/40R18 vs 275/30R19

275/30R19 is 0.09 in (2.4 mm) taller than 235/40R18, a +0.37% diameter difference. With the speedometer set for 235/40R18, an indicated 60 mph is 60.2 mph on 275/30R19.

Is this a safe swap?

Diameter difference +0.37%: under 1%

By overall diameter alone, 275/30R19 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 275/30R19 differences
Measurement	235/40R18	275/30R19	Difference	% diff
Overall diameter	25.40 in (645.2 mm)	25.50 in (647.6 mm)	+0.09 in (+2.4 mm)	+0.37%
Sidewall height	3.70 in (94.0 mm)	3.25 in (82.5 mm)	−0.45 in (−11.5 mm)	−12.23%
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)	80.10 in (2,034.5 mm)	+0.30 in (+7.5 mm)	+0.37%
Revolutions per mile	794	791	−3	−0.37%

True speed with the speedometer calibrated for 235/40R18
Indicated	Actual on 235/40R18	Actual on 275/30R19	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: 275/30R19 raises the vehicle by 0.05 in (1.2 mm) at the axle (half the +2.4 mm diameter difference).

275/30R19 is 0.09 in taller than 235/40R18 and 40 mm wider, a 0.37% difference in overall diameter.

With the speedometer calibrated for 235/40R18, an indicated 60 mph is actually 60.2 mph on 275/30R19, and revolutions per mile change from 794 to 791.

275/30R19 mounts on a 19-inch wheel, a plus-1 fitment from the 18-inch wheel of 235/40R18.

Size pages

Other comparisons with 235/40R18

Other comparisons with 275/30R19

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