Tire Size Box

355/30R20 vs 355/40R20

355/40R20 compared with 355/30R20

+9.8% diameter, 0.0" width

355/40R20 is 31.18" tall vs 28.39". More than the 3% rule of thumb, so check carefully before switching.

28.4"20" rim4.2"14.0" wide
355/30R20 (solid) compared with 355/40R20 (dashed outline), drawn to scale

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

65.9 mph

355/40R20 is +2.80 inches (+9.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/30R20355/40R20Difference%
Overall diameter28.39"31.18"+2.80"+9.8%
Section width13.98"13.98"0.00"0.0%
Sidewall height4.19"5.59"+1.40"+33.3%
Circumference89.18"97.96"+8.78"+9.8%
Revolutions per mile710.5646.8−63.7−9.0%

Speedometer difference

If your vehicle came with 355/30R20 and you fit 355/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph22.0 mph+2.0 mph
30 mph33.0 mph+3.0 mph
40 mph43.9 mph+3.9 mph
50 mph54.9 mph+4.9 mph
60 mph65.9 mph+5.9 mph
65 mph71.4 mph+6.4 mph
70 mph76.9 mph+6.9 mph
75 mph82.4 mph+7.4 mph
80 mph87.9 mph+7.9 mph

Each size on its own

Questions

Which is bigger, 355/30R20 or 355/40R20?
355/40R20 is taller by 2.80 inches (31.18" vs 28.39"). 355/30R20 is the same width.
Can I replace 355/30R20 with 355/40R20?
It is not recommended without checking carefully. The diameter difference is +9.8%, 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 355/30R20), your real speed is 65.9 mph. Your odometer will under-count miles by about 9.8%.
Will 355/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.40 inches of ground clearance at the axle.

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