235/40R18 vs 245/40R18

245/40R18 is 0.31 in (8.0 mm) taller than 235/40R18, a +1.24% diameter difference. With the speedometer set for 235/40R18, an indicated 60 mph is 60.7 mph on 245/40R18.

Is this a safe swap?

Diameter difference +1.24%: within the ±3% guideline

By overall diameter alone, 245/40R18 falls inside the ±3% range commonly used as a guideline when replacing 235/40R18. 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 245/40R18 differences
Measurement	235/40R18	245/40R18	Difference	% diff
Overall diameter	25.40 in (645.2 mm)	25.72 in (653.2 mm)	+0.31 in (+8.0 mm)	+1.24%
Sidewall height	3.70 in (94.0 mm)	3.86 in (98.0 mm)	+0.16 in (+4.0 mm)	+4.26%
Section width	9.25 in (235.0 mm)	9.65 in (245.0 mm)	+0.39 in (+10.0 mm)	+4.26%
Circumference	79.80 in (2,027.0 mm)	80.79 in (2,052.1 mm)	+0.99 in (+25.1 mm)	+1.24%
Revolutions per mile	794	784	−10	−1.22%

True speed with the speedometer calibrated for 235/40R18
Indicated	Actual on 235/40R18	Actual on 245/40R18	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.9 mph	+0.9 mph

Ride-height change: 245/40R18 raises the vehicle by 0.16 in (4.0 mm) at the axle (half the +8.0 mm diameter difference).

245/40R18 is 0.31 in taller than 235/40R18 and 10 mm wider, a 1.24% difference in overall diameter.

With the speedometer calibrated for 235/40R18, an indicated 60 mph is actually 60.7 mph on 245/40R18, and revolutions per mile change from 794 to 784.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/40R18

Other comparisons with 245/40R18

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