Tire Size Box

345/35R20 vs 345/40R20

345/40R20 compared with 345/35R20

+4.6% diameter, 0.0" width

345/40R20 is 30.87" tall vs 29.51". More than the 3% rule of thumb, so check carefully before switching.

29.5"20" rim4.8"13.6" wide
345/35R20 (solid) compared with 345/40R20 (dashed outline), drawn to scale

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

62.8 mph

345/40R20 is +1.36 inches (+4.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/35R20345/40R20Difference%
Overall diameter29.51"30.87"+1.36"+4.6%
Section width13.58"13.58"0.00"0.0%
Sidewall height4.75"5.43"+0.68"+14.3%
Circumference92.70"96.97"+4.27"+4.6%
Revolutions per mile683.5653.4−30.1−4.4%

Speedometer difference

If your vehicle came with 345/35R20 and you fit 345/40R20, 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 mph41.8 mph+1.8 mph
50 mph52.3 mph+2.3 mph
60 mph62.8 mph+2.8 mph
65 mph68.0 mph+3.0 mph
70 mph73.2 mph+3.2 mph
75 mph78.5 mph+3.5 mph
80 mph83.7 mph+3.7 mph

Each size on its own

Questions

Which is bigger, 345/35R20 or 345/40R20?
345/40R20 is taller by 1.36 inches (30.87" vs 29.51"). 345/35R20 is the same width.
Can I replace 345/35R20 with 345/40R20?
It is not recommended without checking carefully. The diameter difference is +4.6%, 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 345/35R20), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.6%.
Will 345/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.68 inches of ground clearance at the axle.

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