Tire Size Box

315/35R20 vs 315/45R20

315/45R20 compared with 315/35R20

+8.6% diameter, 0.0" width

315/45R20 is 31.16" tall vs 28.68". More than the 3% rule of thumb, so check carefully before switching.

28.7"20" rim4.3"12.4" wide
315/35R20 (solid) compared with 315/45R20 (dashed outline), drawn to scale

With 315/45R20 tires, when your speedometer says 60 mph you are really going

65.2 mph

315/45R20 is +2.48 inches (+8.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R20315/45R20Difference%
Overall diameter28.68"31.16"+2.48"+8.6%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.34"5.58"+1.24"+28.6%
Circumference90.10"97.90"+7.79"+8.6%
Revolutions per mile703.2647.2−56.0−8.0%

Speedometer difference

If your vehicle came with 315/35R20 and you fit 315/45R20, 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 mph43.5 mph+3.5 mph
50 mph54.3 mph+4.3 mph
60 mph65.2 mph+5.2 mph
65 mph70.6 mph+5.6 mph
70 mph76.1 mph+6.1 mph
75 mph81.5 mph+6.5 mph
80 mph86.9 mph+6.9 mph

Each size on its own

Questions

Which is bigger, 315/35R20 or 315/45R20?
315/45R20 is taller by 2.48 inches (31.16" vs 28.68"). 315/35R20 is the same width.
Can I replace 315/35R20 with 315/45R20?
It is not recommended without checking carefully. The diameter difference is +8.6%, 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 315/45R20 change my speedometer?
When your speedometer reads 60 mph with 315/45R20 fitted (calibrated for 315/35R20), your real speed is 65.2 mph. Your odometer will under-count miles by about 8.6%.
Will 315/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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