Tire Size Box

325/45R18 vs 325/50R18

325/50R18 compared with 325/45R18

+4.3% diameter, 0.0" width

325/50R18 is 30.80" tall vs 29.52". More than the 3% rule of thumb, so check carefully before switching.

29.5"18" rim5.8"12.8" wide
325/45R18 (solid) compared with 325/50R18 (dashed outline), drawn to scale

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

62.6 mph

325/50R18 is +1.28 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/45R18325/50R18Difference%
Overall diameter29.52"30.80"+1.28"+4.3%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.76"6.40"+0.64"+11.1%
Circumference92.73"96.75"+4.02"+4.3%
Revolutions per mile683.3654.9−28.4−4.2%

Speedometer difference

If your vehicle came with 325/45R18 and you fit 325/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 325/45R18 or 325/50R18?
325/50R18 is taller by 1.28 inches (30.80" vs 29.52"). 325/45R18 is the same width.
Can I replace 325/45R18 with 325/50R18?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 325/45R18), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 325/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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