Tire Size Box

225/40R18 vs 225/50R18

225/50R18 compared with 225/40R18

+7.1% diameter, 0.0" width

225/50R18 is 26.86" tall vs 25.09". More than the 3% rule of thumb, so check carefully before switching.

25.1"18" rim3.5"8.9" wide
225/40R18 (solid) compared with 225/50R18 (dashed outline), drawn to scale

With 225/50R18 tires, when your speedometer says 60 mph you are really going

64.2 mph

225/50R18 is +1.77 inches (+7.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/40R18225/50R18Difference%
Overall diameter25.09"26.86"+1.77"+7.1%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.54"4.43"+0.89"+25.0%
Circumference78.81"84.38"+5.57"+7.1%
Revolutions per mile803.9750.9−53.0−6.6%

Speedometer difference

If your vehicle came with 225/40R18 and you fit 225/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.1 mph+2.1 mph
40 mph42.8 mph+2.8 mph
50 mph53.5 mph+3.5 mph
60 mph64.2 mph+4.2 mph
65 mph69.6 mph+4.6 mph
70 mph74.9 mph+4.9 mph
75 mph80.3 mph+5.3 mph
80 mph85.6 mph+5.6 mph

Each size on its own

Questions

Which is bigger, 225/40R18 or 225/50R18?
225/50R18 is taller by 1.77 inches (26.86" vs 25.09"). 225/40R18 is the same width.
Can I replace 225/40R18 with 225/50R18?
It is not recommended without checking carefully. The diameter difference is +7.1%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 225/50R18 change my speedometer?
When your speedometer reads 60 mph with 225/50R18 fitted (calibrated for 225/40R18), your real speed is 64.2 mph. Your odometer will under-count miles by about 7.1%.
Will 225/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.89 inches of ground clearance at the axle.

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