Tire Size Box

325/35R20 vs 325/45R20

325/45R20 compared with 325/35R20

+8.8% diameter, 0.0" width

325/45R20 is 31.52" tall vs 28.96". More than the 3% rule of thumb, so check carefully before switching.

29.0"20" rim4.5"12.8" wide
325/35R20 (solid) compared with 325/45R20 (dashed outline), drawn to scale

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

65.3 mph

325/45R20 is +2.56 inches (+8.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R20325/45R20Difference%
Overall diameter28.96"31.52"+2.56"+8.8%
Section width12.80"12.80"0.00"0.0%
Sidewall height4.48"5.76"+1.28"+28.6%
Circumference90.97"99.01"+8.04"+8.8%
Revolutions per mile696.5639.9−56.6−8.1%

Speedometer difference

If your vehicle came with 325/35R20 and you fit 325/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.5 mph+3.5 mph
50 mph54.4 mph+4.4 mph
60 mph65.3 mph+5.3 mph
65 mph70.7 mph+5.7 mph
70 mph76.2 mph+6.2 mph
75 mph81.6 mph+6.6 mph
80 mph87.1 mph+7.1 mph

Each size on its own

Questions

Which is bigger, 325/35R20 or 325/45R20?
325/45R20 is taller by 2.56 inches (31.52" vs 28.96"). 325/35R20 is the same width.
Can I replace 325/35R20 with 325/45R20?
It is not recommended without checking carefully. The diameter difference is +8.8%, 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 325/35R20), your real speed is 65.3 mph. Your odometer will under-count miles by about 8.8%.
Will 325/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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