225/40R18 vs 255/30R19

255/30R19 is 0.06 in (1.6 mm) shorter than 225/40R18, a −0.25% diameter difference. With the speedometer set for 225/40R18, an indicated 60 mph is 59.8 mph on 255/30R19.

Is this a safe swap?

Diameter difference −0.25%: under 1%

By overall diameter alone, 255/30R19 is a near match for 225/40R18; 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.

225/40R18 vs 255/30R19 differences
Measurement	225/40R18	255/30R19	Difference	% diff
Overall diameter	25.09 in (637.2 mm)	25.02 in (635.6 mm)	−0.06 in (−1.6 mm)	−0.25%
Sidewall height	3.54 in (90.0 mm)	3.01 in (76.5 mm)	−0.53 in (−13.5 mm)	−15.00%
Section width	8.86 in (225.0 mm)	10.04 in (255.0 mm)	+1.18 in (+30.0 mm)	+13.33%
Circumference	78.81 in (2,001.8 mm)	78.61 in (1,996.8 mm)	−0.20 in (−5.0 mm)	−0.25%
Revolutions per mile	804	806	+2	+0.25%

True speed with the speedometer calibrated for 225/40R18
Indicated	Actual on 225/40R18	Actual on 255/30R19	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.8 mph	−0.2 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 255/30R19 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

255/30R19 is 0.06 in shorter than 225/40R18 and 30 mm wider, a 0.25% difference in overall diameter.

With the speedometer calibrated for 225/40R18, an indicated 60 mph is actually 59.8 mph on 255/30R19, and revolutions per mile change from 804 to 806.

255/30R19 mounts on a 19-inch wheel, a plus-1 fitment from the 18-inch wheel of 225/40R18.

Size pages

Other comparisons with 225/40R18

Other comparisons with 255/30R19

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