Tire Size Box

355/30R21 vs 355/40R21

355/40R21 compared with 355/30R21

+9.5% diameter, 0.0" width

355/40R21 is 32.18" tall vs 29.39". More than the 3% rule of thumb, so check carefully before switching.

29.4"21" rim4.2"14.0" wide
355/30R21 (solid) compared with 355/40R21 (dashed outline), drawn to scale

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

65.7 mph

355/40R21 is +2.80 inches (+9.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/30R21355/40R21Difference%
Overall diameter29.39"32.18"+2.80"+9.5%
Section width13.98"13.98"0.00"0.0%
Sidewall height4.19"5.59"+1.40"+33.3%
Circumference92.32"101.10"+8.78"+9.5%
Revolutions per mile686.3626.7−59.6−8.7%

Speedometer difference

If your vehicle came with 355/30R21 and you fit 355/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.9 mph+1.9 mph
30 mph32.9 mph+2.9 mph
40 mph43.8 mph+3.8 mph
50 mph54.8 mph+4.8 mph
60 mph65.7 mph+5.7 mph
65 mph71.2 mph+6.2 mph
70 mph76.7 mph+6.7 mph
75 mph82.1 mph+7.1 mph
80 mph87.6 mph+7.6 mph

Each size on its own

Questions

Which is bigger, 355/30R21 or 355/40R21?
355/40R21 is taller by 2.80 inches (32.18" vs 29.39"). 355/30R21 is the same width.
Can I replace 355/30R21 with 355/40R21?
It is not recommended without checking carefully. The diameter difference is +9.5%, 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 355/30R21), your real speed is 65.7 mph. Your odometer will under-count miles by about 9.5%.
Will 355/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.40 inches of ground clearance at the axle.

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