Tire Size Box

325/30R24 vs 325/40R24

325/40R24 compared with 325/30R24

+8.1% diameter, 0.0" width

325/40R24 is 34.24" tall vs 31.68". More than the 3% rule of thumb, so check carefully before switching.

31.7"24" rim3.8"12.8" wide
325/30R24 (solid) compared with 325/40R24 (dashed outline), drawn to scale

With 325/40R24 tires, when your speedometer says 60 mph you are really going

64.8 mph

325/40R24 is +2.56 inches (+8.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/30R24325/40R24Difference%
Overall diameter31.68"34.24"+2.56"+8.1%
Section width12.80"12.80"0.00"0.0%
Sidewall height3.84"5.12"+1.28"+33.3%
Circumference99.52"107.56"+8.04"+8.1%
Revolutions per mile636.7589.1−47.6−7.5%

Speedometer difference

If your vehicle came with 325/30R24 and you fit 325/40R24, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.5 mph+6.5 mph

Each size on its own

Questions

Which is bigger, 325/30R24 or 325/40R24?
325/40R24 is taller by 2.56 inches (34.24" vs 31.68"). 325/30R24 is the same width.
Can I replace 325/30R24 with 325/40R24?
It is not recommended without checking carefully. The diameter difference is +8.1%, 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 325/40R24 change my speedometer?
When your speedometer reads 60 mph with 325/40R24 fitted (calibrated for 325/30R24), your real speed is 64.8 mph. Your odometer will under-count miles by about 8.1%.
Will 325/40R24 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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