Tire Size Box

335/30R21 vs 335/40R21

335/40R21 compared with 335/30R21

+9.1% diameter, 0.0" width

335/40R21 is 31.55" tall vs 28.91". More than the 3% rule of thumb, so check carefully before switching.

28.9"21" rim4.0"13.2" wide
335/30R21 (solid) compared with 335/40R21 (dashed outline), drawn to scale

With 335/40R21 tires, when your speedometer says 60 mph you are really going

65.5 mph

335/40R21 is +2.64 inches (+9.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

335/30R21335/40R21Difference%
Overall diameter28.91"31.55"+2.64"+9.1%
Section width13.19"13.19"0.00"0.0%
Sidewall height3.96"5.28"+1.32"+33.3%
Circumference90.83"99.12"+8.29"+9.1%
Revolutions per mile697.5639.2−58.3−8.4%

Speedometer difference

If your vehicle came with 335/30R21 and you fit 335/40R21, 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 mph54.6 mph+4.6 mph
60 mph65.5 mph+5.5 mph
65 mph70.9 mph+5.9 mph
70 mph76.4 mph+6.4 mph
75 mph81.8 mph+6.8 mph
80 mph87.3 mph+7.3 mph

Each size on its own

Questions

Which is bigger, 335/30R21 or 335/40R21?
335/40R21 is taller by 2.64 inches (31.55" vs 28.91"). 335/30R21 is the same width.
Can I replace 335/30R21 with 335/40R21?
It is not recommended without checking carefully. The diameter difference is +9.1%, 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 335/40R21 change my speedometer?
When your speedometer reads 60 mph with 335/40R21 fitted (calibrated for 335/30R21), your real speed is 65.5 mph. Your odometer will under-count miles by about 9.1%.
Will 335/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.32 inches of ground clearance at the axle.

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