Tire Size Box

315/45R20 vs 315/55R20

315/55R20 compared with 315/45R20

+8.0% diameter, 0.0" width

315/55R20 is 33.64" tall vs 31.16". More than the 3% rule of thumb, so check carefully before switching.

31.2"20" rim5.6"12.4" wide
315/45R20 (solid) compared with 315/55R20 (dashed outline), drawn to scale

With 315/55R20 tires, when your speedometer says 60 mph you are really going

64.8 mph

315/55R20 is +2.48 inches (+8.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/45R20315/55R20Difference%
Overall diameter31.16"33.64"+2.48"+8.0%
Section width12.40"12.40"0.00"0.0%
Sidewall height5.58"6.82"+1.24"+22.2%
Circumference97.90"105.69"+7.79"+8.0%
Revolutions per mile647.2599.5−47.7−7.4%

Speedometer difference

If your vehicle came with 315/45R20 and you fit 315/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.2 mph+5.2 mph
70 mph75.6 mph+5.6 mph
75 mph81.0 mph+6.0 mph
80 mph86.4 mph+6.4 mph

Each size on its own

Questions

Which is bigger, 315/45R20 or 315/55R20?
315/55R20 is taller by 2.48 inches (33.64" vs 31.16"). 315/45R20 is the same width.
Can I replace 315/45R20 with 315/55R20?
It is not recommended without checking carefully. The diameter difference is +8.0%, 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 315/55R20 change my speedometer?
When your speedometer reads 60 mph with 315/55R20 fitted (calibrated for 315/45R20), your real speed is 64.8 mph. Your odometer will under-count miles by about 8.0%.
Will 315/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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