Tire Size Box

325/35R21 vs 325/45R21

325/45R21 compared with 325/35R21

+8.5% diameter, 0.0" width

325/45R21 is 32.52" tall vs 29.96". More than the 3% rule of thumb, so check carefully before switching.

30.0"21" rim4.5"12.8" wide
325/35R21 (solid) compared with 325/45R21 (dashed outline), drawn to scale

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

65.1 mph

325/45R21 is +2.56 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R21325/45R21Difference%
Overall diameter29.96"32.52"+2.56"+8.5%
Section width12.80"12.80"0.00"0.0%
Sidewall height4.48"5.76"+1.28"+28.6%
Circumference94.11"102.15"+8.04"+8.5%
Revolutions per mile673.2620.3−53.0−7.9%

Speedometer difference

If your vehicle came with 325/35R21 and you fit 325/45R21, 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 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.1 mph+5.1 mph
65 mph70.6 mph+5.6 mph
70 mph76.0 mph+6.0 mph
75 mph81.4 mph+6.4 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 325/35R21 or 325/45R21?
325/45R21 is taller by 2.56 inches (32.52" vs 29.96"). 325/35R21 is the same width.
Can I replace 325/35R21 with 325/45R21?
It is not recommended without checking carefully. The diameter difference is +8.5%, 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 325/35R21), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 325/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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