Tire Size Box

355/35R20 vs 355/40R20

355/40R20 compared with 355/35R20

+4.7% diameter, 0.0" width

355/40R20 is 31.18" tall vs 29.78". More than the 3% rule of thumb, so check carefully before switching.

29.8"20" rim4.9"14.0" wide
355/35R20 (solid) compared with 355/40R20 (dashed outline), drawn to scale

With 355/40R20 tires, when your speedometer says 60 mph you are really going

62.8 mph

355/40R20 is +1.40 inches (+4.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/35R20355/40R20Difference%
Overall diameter29.78"31.18"+1.40"+4.7%
Section width13.98"13.98"0.00"0.0%
Sidewall height4.89"5.59"+0.70"+14.3%
Circumference93.57"97.96"+4.39"+4.7%
Revolutions per mile677.2646.8−30.4−4.5%

Speedometer difference

If your vehicle came with 355/35R20 and you fit 355/40R20, 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 mph41.9 mph+1.9 mph
50 mph52.3 mph+2.3 mph
60 mph62.8 mph+2.8 mph
65 mph68.1 mph+3.1 mph
70 mph73.3 mph+3.3 mph
75 mph78.5 mph+3.5 mph
80 mph83.8 mph+3.8 mph

Each size on its own

Questions

Which is bigger, 355/35R20 or 355/40R20?
355/40R20 is taller by 1.40 inches (31.18" vs 29.78"). 355/35R20 is the same width.
Can I replace 355/35R20 with 355/40R20?
It is not recommended without checking carefully. The diameter difference is +4.7%, 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 355/40R20 change my speedometer?
When your speedometer reads 60 mph with 355/40R20 fitted (calibrated for 355/35R20), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.7%.
Will 355/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.70 inches of ground clearance at the axle.

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