Tire Size Box

325/40R20 vs 325/45R20

325/45R20 compared with 325/40R20

+4.2% diameter, 0.0" width

325/45R20 is 31.52" tall vs 30.24". More than the 3% rule of thumb, so check carefully before switching.

30.2"20" rim5.1"12.8" wide
325/40R20 (solid) compared with 325/45R20 (dashed outline), drawn to scale

With 325/45R20 tires, when your speedometer says 60 mph you are really going

62.5 mph

325/45R20 is +1.28 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R20325/45R20Difference%
Overall diameter30.24"31.52"+1.28"+4.2%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.12"5.76"+0.64"+12.5%
Circumference94.99"99.01"+4.02"+4.2%
Revolutions per mile667.0639.9−27.1−4.1%

Speedometer difference

If your vehicle came with 325/40R20 and you fit 325/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 325/40R20 or 325/45R20?
325/45R20 is taller by 1.28 inches (31.52" vs 30.24"). 325/40R20 is the same width.
Can I replace 325/40R20 with 325/45R20?
It is not recommended without checking carefully. The diameter difference is +4.2%, 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 325/45R20 change my speedometer?
When your speedometer reads 60 mph with 325/45R20 fitted (calibrated for 325/40R20), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 325/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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