Tire Size Box

325/40R18 vs 325/50R18

325/50R18 compared with 325/40R18

+9.1% diameter, 0.0" width

325/50R18 is 30.80" tall vs 28.24". More than the 3% rule of thumb, so check carefully before switching.

28.2"18" rim5.1"12.8" wide
325/40R18 (solid) compared with 325/50R18 (dashed outline), drawn to scale

With 325/50R18 tires, when your speedometer says 60 mph you are really going

65.4 mph

325/50R18 is +2.56 inches (+9.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R18325/50R18Difference%
Overall diameter28.24"30.80"+2.56"+9.1%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.12"6.40"+1.28"+25.0%
Circumference88.71"96.75"+8.04"+9.1%
Revolutions per mile714.3654.9−59.4−8.3%

Speedometer difference

If your vehicle came with 325/40R18 and you fit 325/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.5 mph+4.5 mph
60 mph65.4 mph+5.4 mph
65 mph70.9 mph+5.9 mph
70 mph76.3 mph+6.3 mph
75 mph81.8 mph+6.8 mph
80 mph87.3 mph+7.3 mph

Each size on its own

Questions

Which is bigger, 325/40R18 or 325/50R18?
325/50R18 is taller by 2.56 inches (30.80" vs 28.24"). 325/40R18 is the same width.
Can I replace 325/40R18 with 325/50R18?
It is not recommended without checking carefully. The diameter difference is +9.1%, 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 325/50R18 change my speedometer?
When your speedometer reads 60 mph with 325/50R18 fitted (calibrated for 325/40R18), your real speed is 65.4 mph. Your odometer will under-count miles by about 9.1%.
Will 325/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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