Tire Size Box

315/45R21 vs 315/55R21

315/55R21 compared with 315/45R21

+7.7% diameter, 0.0" width

315/55R21 is 34.64" tall vs 32.16". More than the 3% rule of thumb, so check carefully before switching.

32.2"21" rim5.6"12.4" wide
315/45R21 (solid) compared with 315/55R21 (dashed outline), drawn to scale

With 315/55R21 tires, when your speedometer says 60 mph you are really going

64.6 mph

315/55R21 is +2.48 inches (+7.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/45R21315/55R21Difference%
Overall diameter32.16"34.64"+2.48"+7.7%
Section width12.40"12.40"0.00"0.0%
Sidewall height5.58"6.82"+1.24"+22.2%
Circumference101.04"108.83"+7.79"+7.7%
Revolutions per mile627.1582.2−44.9−7.2%

Speedometer difference

If your vehicle came with 315/45R21 and you fit 315/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.6 mph+4.6 mph
65 mph70.0 mph+5.0 mph
70 mph75.4 mph+5.4 mph
75 mph80.8 mph+5.8 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 315/45R21 or 315/55R21?
315/55R21 is taller by 2.48 inches (34.64" vs 32.16"). 315/45R21 is the same width.
Can I replace 315/45R21 with 315/55R21?
It is not recommended without checking carefully. The diameter difference is +7.7%, 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 315/55R21 change my speedometer?
When your speedometer reads 60 mph with 315/55R21 fitted (calibrated for 315/45R21), your real speed is 64.6 mph. Your odometer will under-count miles by about 7.7%.
Will 315/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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