Tire Size Box

355/35R20 vs 345/40R20

345/40R20 compared with 355/35R20

+3.6% diameter, −0.4" width

345/40R20 is 30.87" 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 345/40R20 (dashed outline), drawn to scale

With 345/40R20 tires, when your speedometer says 60 mph you are really going

62.2 mph

345/40R20 is +1.08 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/35R20345/40R20Difference%
Overall diameter29.78"30.87"+1.08"+3.6%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height4.89"5.43"+0.54"+11.1%
Circumference93.57"96.97"+3.40"+3.6%
Revolutions per mile677.2653.4−23.8−3.5%

Speedometer difference

If your vehicle came with 355/35R20 and you fit 345/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 355/35R20 or 345/40R20?
345/40R20 is taller by 1.08 inches (30.87" vs 29.78"). 355/35R20 is wider by 0.39 inches.
Can I replace 355/35R20 with 345/40R20?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 345/40R20 change my speedometer?
When your speedometer reads 60 mph with 345/40R20 fitted (calibrated for 355/35R20), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 345/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

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