Tire Size Box

325/35R20 vs 325/40R20

325/40R20 compared with 325/35R20

+4.4% diameter, 0.0" width

325/40R20 is 30.24" 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 325/40R20 (dashed outline), drawn to scale

With 325/40R20 tires, when your speedometer says 60 mph you are really going

62.7 mph

325/40R20 is +1.28 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R20325/40R20Difference%
Overall diameter28.96"30.24"+1.28"+4.4%
Section width12.80"12.80"0.00"0.0%
Sidewall height4.48"5.12"+0.64"+14.3%
Circumference90.97"94.99"+4.02"+4.4%
Revolutions per mile696.5667.0−29.5−4.2%

Speedometer difference

If your vehicle came with 325/35R20 and you fit 325/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.8 mph+1.8 mph
50 mph52.2 mph+2.2 mph
60 mph62.7 mph+2.7 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 325/35R20 or 325/40R20?
325/40R20 is taller by 1.28 inches (30.24" vs 28.96"). 325/35R20 is the same width.
Can I replace 325/35R20 with 325/40R20?
It is not recommended without checking carefully. The diameter difference is +4.4%, 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 325/40R20 change my speedometer?
When your speedometer reads 60 mph with 325/40R20 fitted (calibrated for 325/35R20), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.4%.
Will 325/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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