Tire Size Box

315/45R18 vs 315/55R18

315/55R18 compared with 315/45R18

+8.5% diameter, 0.0" width

315/55R18 is 31.64" tall vs 29.16". More than the 3% rule of thumb, so check carefully before switching.

29.2"18" rim5.6"12.4" wide
315/45R18 (solid) compared with 315/55R18 (dashed outline), drawn to scale

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

65.1 mph

315/55R18 is +2.48 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/45R18315/55R18Difference%
Overall diameter29.16"31.64"+2.48"+8.5%
Section width12.40"12.40"0.00"0.0%
Sidewall height5.58"6.82"+1.24"+22.2%
Circumference91.61"99.41"+7.79"+8.5%
Revolutions per mile691.6637.4−54.2−7.8%

Speedometer difference

If your vehicle came with 315/45R18 and you fit 315/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph76.0 mph+6.0 mph
75 mph81.4 mph+6.4 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 315/45R18 or 315/55R18?
315/55R18 is taller by 2.48 inches (31.64" vs 29.16"). 315/45R18 is the same width.
Can I replace 315/45R18 with 315/55R18?
It is not recommended without checking carefully. The diameter difference is +8.5%, 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 315/45R18), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 315/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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