Tire Size Box

355/35R21 vs 355/40R21

355/40R21 compared with 355/35R21

+4.5% diameter, 0.0" width

355/40R21 is 32.18" tall vs 30.78". More than the 3% rule of thumb, so check carefully before switching.

30.8"21" rim4.9"14.0" wide
355/35R21 (solid) compared with 355/40R21 (dashed outline), drawn to scale

With 355/40R21 tires, when your speedometer says 60 mph you are really going

62.7 mph

355/40R21 is +1.40 inches (+4.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/35R21355/40R21Difference%
Overall diameter30.78"32.18"+1.40"+4.5%
Section width13.98"13.98"0.00"0.0%
Sidewall height4.89"5.59"+0.70"+14.3%
Circumference96.71"101.10"+4.39"+4.5%
Revolutions per mile655.2626.7−28.5−4.3%

Speedometer difference

If your vehicle came with 355/35R21 and you fit 355/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.8 mph+1.8 mph
50 mph52.3 mph+2.3 mph
60 mph62.7 mph+2.7 mph
65 mph68.0 mph+3.0 mph
70 mph73.2 mph+3.2 mph
75 mph78.4 mph+3.4 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 355/35R21 or 355/40R21?
355/40R21 is taller by 1.40 inches (32.18" vs 30.78"). 355/35R21 is the same width.
Can I replace 355/35R21 with 355/40R21?
It is not recommended without checking carefully. The diameter difference is +4.5%, 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 355/40R21 change my speedometer?
When your speedometer reads 60 mph with 355/40R21 fitted (calibrated for 355/35R21), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.5%.
Will 355/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.70 inches of ground clearance at the axle.

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