The speed sensor doesn't care what gear you are in, it measure RP M's of the output shaft, one revolution equals so many rpms of the drive tires, it is fixed by rear gears and tire size, the speedometer still works if you coast out of gear. if you don't change rear gears or tire size no adjustment is necessary.

Adding a aux transmission will add drag to the drive line, more drag more fuel. You can gain more fuel mileage by tweaking ECM settings, increasing fuel flow and changing to free flowing mufflers, it also cost a lot less and your truck will run better and have more power.

 

Correct if you only have 1 transmission

In this case, with the VSS on the output shaft of the 13 speed, if you were to coast with the aux. trans in neutral, the speedometer would read zero.

With the VSS on the output shaft of the 13 speed you would effectively be changing the final drive ratio whenever the aux. trans is in any gear except direct.

In his stock configuration, the VSS is on the output shaft of the 13 speed.

If you put an auxiliary transmission behind the 13 speed and leave the VSS where it is, the only time the truck's speedometer will read accurately is when the aux. is in direct. When the aux. is in an underdrive or overdrive ratio, the truck will go slower or faster, respectively, rendering the speedometer inaccurate.

The solution to this is to move the VSS to the output shaft of the auxiliary.


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That is where it needs to be.

I don't believe you could gain enough with a Aux trans to recoup the cost, with no other changes I believe fuel mileage would go down.
If you are after mileage go direct drive trans, 2.xx gears, single rear with a tag axle, super singles and aero as much as possible.

 

As of January 2010 the speed limit for trucks in Illinois will be the same as for cars - 65 MPH.

 

Yes, I know.

Direct drive trans and 2.xx rear gears would work great for top end because there would be less parasitic H.P. loss with no overdrive gearing in the equation.

The problem would be that by gearing the rear axle that high you would not have enough overall reduction on the bottom end, i.e. with the trans in low gear on a steep hill or in soft ground.

That's one of my pet peeves with modern day truck transmissions: there is not enough reduction in low gear to get moving unless you spec a low geared (higher numerical ratio) rear axle ratio, and if you do that, you effectively limit your top road speed because the truck is "gearbound".

Also as I stated before, the driveshaft and U joints would be under extreme stress in a low gear severe start situation with 2.xx rear gears.

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I want to thank everyone that has given me there input, it has given me a lot to think about.

if truck tires are only rated at 75 mph then that's going to be my top speed goal.
I really need to learn all the formula's for figuring out gear ratio's and put them in a spreadsheet so I can play with the numbers before I jump on such a major change to my truck.

Again thank you all for your time and input!!

 

So what happens after you spend 10K and don't gain anything? Get professional help.