Tire Size Box

325/30R21 vs 325/40R21

325/40R21 compared with 325/30R21

+8.9% diameter, 0.0" width

325/40R21 is 31.24" tall vs 28.68". More than the 3% rule of thumb, so check carefully before switching.

28.7"21" rim3.8"12.8" wide
325/30R21 (solid) compared with 325/40R21 (dashed outline), drawn to scale

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

65.4 mph

325/40R21 is +2.56 inches (+8.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/30R21325/40R21Difference%
Overall diameter28.68"31.24"+2.56"+8.9%
Section width12.80"12.80"0.00"0.0%
Sidewall height3.84"5.12"+1.28"+33.3%
Circumference90.09"98.13"+8.04"+8.9%
Revolutions per mile703.3645.7−57.6−8.2%

Speedometer difference

If your vehicle came with 325/30R21 and you fit 325/40R21, 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 mph43.6 mph+3.6 mph
50 mph54.5 mph+4.5 mph
60 mph65.4 mph+5.4 mph
65 mph70.8 mph+5.8 mph
70 mph76.2 mph+6.2 mph
75 mph81.7 mph+6.7 mph
80 mph87.1 mph+7.1 mph

Each size on its own

Questions

Which is bigger, 325/30R21 or 325/40R21?
325/40R21 is taller by 2.56 inches (31.24" vs 28.68"). 325/30R21 is the same width.
Can I replace 325/30R21 with 325/40R21?
It is not recommended without checking carefully. The diameter difference is +8.9%, 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 325/30R21), your real speed is 65.4 mph. Your odometer will under-count miles by about 8.9%.
Will 325/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

Other comparisons