Tire Size Box

325/45R18 vs 315/50R18

315/50R18 compared with 325/45R18

+3.0% diameter, −0.4" width

315/50R18 is 30.40" 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 315/50R18 (dashed outline), drawn to scale

With 315/50R18 tires, when your speedometer says 60 mph you are really going

61.8 mph

315/50R18 is +0.89 inches (+3.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/45R18315/50R18Difference%
Overall diameter29.52"30.40"+0.89"+3.0%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height5.76"6.20"+0.44"+7.7%
Circumference92.73"95.51"+2.78"+3.0%
Revolutions per mile683.3663.4−19.9−2.9%

Speedometer difference

If your vehicle came with 325/45R18 and you fit 315/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.4 mph+2.4 mph

Each size on its own

Questions

Which is bigger, 325/45R18 or 315/50R18?
315/50R18 is taller by 0.89 inches (30.40" vs 29.52"). 325/45R18 is wider by 0.39 inches.
Can I replace 325/45R18 with 315/50R18?
It is not recommended without checking carefully. The diameter difference is +3.0%, 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 315/50R18 change my speedometer?
When your speedometer reads 60 mph with 315/50R18 fitted (calibrated for 325/45R18), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.0%.
Will 315/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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