Tire Size Box

325/40R21 vs 325/45R21

325/45R21 compared with 325/40R21

+4.1% diameter, 0.0" width

325/45R21 is 32.52" tall vs 31.24". More than the 3% rule of thumb, so check carefully before switching.

31.2"21" rim5.1"12.8" wide
325/40R21 (solid) compared with 325/45R21 (dashed outline), drawn to scale

With 325/45R21 tires, when your speedometer says 60 mph you are really going

62.5 mph

325/45R21 is +1.28 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R21325/45R21Difference%
Overall diameter31.24"32.52"+1.28"+4.1%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.12"5.76"+0.64"+12.5%
Circumference98.13"102.15"+4.02"+4.1%
Revolutions per mile645.7620.3−25.4−3.9%

Speedometer difference

If your vehicle came with 325/40R21 and you fit 325/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 325/40R21 or 325/45R21?
325/45R21 is taller by 1.28 inches (32.52" vs 31.24"). 325/40R21 is the same width.
Can I replace 325/40R21 with 325/45R21?
It is not recommended without checking carefully. The diameter difference is +4.1%, 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 325/45R21 change my speedometer?
When your speedometer reads 60 mph with 325/45R21 fitted (calibrated for 325/40R21), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.1%.
Will 325/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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