225/50R17 vs 245/40R18

245/40R18 is 0.14 in (3.6 mm) shorter than 225/50R17, a −0.55% diameter difference. With the speedometer set for 225/50R17, an indicated 60 mph is 59.7 mph on 245/40R18.

Is this a safe swap?

Diameter difference −0.55%: under 1%

By overall diameter alone, 245/40R18 is a near match for 225/50R17; 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.

225/50R17 vs 245/40R18 differences
Measurement	225/50R17	245/40R18	Difference	% diff
Overall diameter	25.86 in (656.8 mm)	25.72 in (653.2 mm)	−0.14 in (−3.6 mm)	−0.55%
Sidewall height	4.43 in (112.5 mm)	3.86 in (98.0 mm)	−0.57 in (−14.5 mm)	−12.89%
Section width	8.86 in (225.0 mm)	9.65 in (245.0 mm)	+0.79 in (+20.0 mm)	+8.89%
Circumference	81.24 in (2,063.4 mm)	80.79 in (2,052.1 mm)	−0.45 in (−11.3 mm)	−0.55%
Revolutions per mile	780	784	+4	+0.55%

True speed with the speedometer calibrated for 225/50R17
Indicated	Actual on 225/50R17	Actual on 245/40R18	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 245/40R18 lowers the vehicle by 0.07 in (1.8 mm) at the axle (half the −3.6 mm diameter difference).

245/40R18 is 0.14 in shorter than 225/50R17 and 20 mm wider, a 0.55% difference in overall diameter.

With the speedometer calibrated for 225/50R17, an indicated 60 mph is actually 59.7 mph on 245/40R18, and revolutions per mile change from 780 to 784.

245/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 225/50R17.

Size pages

Other comparisons with 225/50R17

Other comparisons with 245/40R18

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