3:42 rears

if you are doing HH - you need to understand that you go up hill slow and down hill slow. That is part of the deal...

I would like to be able to go up hill ay 65 mph but guess what - it maybe a lot closer to 6.5 mph

and loaded to 130-140k ( which is the minimum you will be hauling a 336 (80k no bucket at least) you should expect 4 mpg or worse

 

Do they make a 3:70?

 

How fast you go up a hill has nothing to do with gearing and everything to do with how much horsepower your engine puts out.
Whether you go up the hill in 18 th with 3.90 or up in 16th with 3.42. You will still climb at the exact same mph.
It only makes a difference in how often you have to shift.
The difference gearing makes is how fast the truck will rev out at, 3.42 would have a higher potential speed. Also how steep of a hill you can readily launch the truck without frying the clutch, 3.90 has better startability.

 

WHY?

All that would do is change the RPM at a given speed.
Does nothing for the gear ratio.
So if all you need is more RPM then shift down, IMO,

 

Changing one tire size is equivalent to one step in rear end ratio.
3.90 with 11R22.5 is about the same as 4.10 with 11R24.5
Or going from 11R24.5 with 3.42 would be similar to a 3.55 if you only changed the tire size.

 

COMMON MISCONCEPTION

Changing tire size does NOT change gear ratio. All that changes is rollout and rpm for a given speed.

What do the numbers mean and how does tire size change that?
Answer - it doesn't'
The torque to the tire remains the same no matter the tire size/height.

 

Technically correct, it is the same force on the axle to the wheel.
One size wheel will give it slightly more leverage than the other. We're only talking about a 5% difference one way or the other. Not enough to justify the expense.