Tire Size Box

325/35R21 vs 325/40R21

325/40R21 compared with 325/35R21

+4.3% diameter, 0.0" width

325/40R21 is 31.24" 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 325/40R21 (dashed outline), drawn to scale

With 325/40R21 tires, when your speedometer says 60 mph you are really going

62.6 mph

325/40R21 is +1.28 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R21325/40R21Difference%
Overall diameter29.96"31.24"+1.28"+4.3%
Section width12.80"12.80"0.00"0.0%
Sidewall height4.48"5.12"+0.64"+14.3%
Circumference94.11"98.13"+4.02"+4.3%
Revolutions per mile673.2645.7−27.6−4.1%

Speedometer difference

If your vehicle came with 325/35R21 and you fit 325/40R21, 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 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.6 mph+2.6 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, 325/35R21 or 325/40R21?
325/40R21 is taller by 1.28 inches (31.24" vs 29.96"). 325/35R21 is the same width.
Can I replace 325/35R21 with 325/40R21?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 325/40R21 change my speedometer?
When your speedometer reads 60 mph with 325/40R21 fitted (calibrated for 325/35R21), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 325/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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