Tire Size Box

315/50R18 vs 315/55R18

315/55R18 compared with 315/50R18

+4.1% diameter, 0.0" width

315/55R18 is 31.64" tall vs 30.40". More than the 3% rule of thumb, so check carefully before switching.

30.4"18" rim6.2"12.4" wide
315/50R18 (solid) compared with 315/55R18 (dashed outline), drawn to scale

With 315/55R18 tires, when your speedometer says 60 mph you are really going

62.4 mph

315/55R18 is +1.24 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/50R18315/55R18Difference%
Overall diameter30.40"31.64"+1.24"+4.1%
Section width12.40"12.40"0.00"0.0%
Sidewall height6.20"6.82"+0.62"+10.0%
Circumference95.51"99.41"+3.90"+4.1%
Revolutions per mile663.4637.4−26.0−3.9%

Speedometer difference

If your vehicle came with 315/50R18 and you fit 315/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.4 mph+2.4 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 315/50R18 or 315/55R18?
315/55R18 is taller by 1.24 inches (31.64" vs 30.40"). 315/50R18 is the same width.
Can I replace 315/50R18 with 315/55R18?
It is not recommended without checking carefully. The diameter difference is +4.1%, 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 315/55R18 change my speedometer?
When your speedometer reads 60 mph with 315/55R18 fitted (calibrated for 315/50R18), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.1%.
Will 315/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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