Here is the is the part where the older trucks have problems.  See the truck was built in 1994, and anyone that was of driving age at that time will remember the speed limit was still 55mph.  The truck was built to get the best power and fuel economy at that speed.  The problem comes when the speed limit was raised to 65mph and in the 5 speed trucks you got 4.11's in the rear.  This all equates to a highway engine rpm of about 2350.  Here is where fuel mileage starts to drop, the powerstroke gets the best fuel economy at peak tourque which is between 1800 and 2200 rpm, spinning the engine any faster than that and fuel mileage goes downhill in a hurry.  So, the quick and easy way to remedy that would be to swap the gears in the rear end to a set of 3.73 or 3.55's.  That gets your engine rpms down to where they were at 55mph with the 4.11's but you start to sacrifice low end pulling power.  Now, if your truck is an automatic, it most likely has a set of 3.55's in the diff already.  The problem is ford was using the E4OD which was built for the gas motors and not the higher torque powerstroke.  Also, there are some pretty serious engine harmonics to deal with in a diesel engine, which is the reason for the dual mass flywheel in the manual trucks and the reason torque converter studs and mounting tabs will get destroyed in the long run.

Again I seem to have a few ideas floating around in my head.  They all have their pros and cons but to nail down one option that is perfect for what I want is rather hard.  So here is what I have for options.

    1.  Keep the factory ZF-5 speed transmission.  While this is not the perfect trans for the application it is hard to discount it's durability.  With a fairly low 1st gear (4.14:1 with the diesel engine) and an average overdrive (.76:1) gearing is about right.  However, I really hate driving around town and shifting all the time.  Also, the clutch is the weak link in this whole deal and let me tell you it is REALLY friggin' expensive.

    2.  Swap in a really stout manual transmission from a medium duty truck with 6-7 or even 10 speeds.  While this might be a good solution to durability issues finding one with any taller overdrive could be a little scarce.  Along with some issues of making the clutch work right.

    3.  Go automatic.  Now there are alot of die hard "gear jammers" out there that will ask "why the hell do you want a slush box in a tow vehicle ?"  Well, the answer is very simple, when built right, there is absolutly no better way to get power to the ground than an automatic.  Why you ask?  Lemme explain, for starters, most people assume the only purpose of a torque converter is to do what the clutch does for a manual transmission which is true to an extent.  However, a torque converter also plays one VERY important role, it acts as a torque multiplier between the engine and transmission.  If you look at the different options for converters in the allison automatic you can get anywhere from a 1.15:1 stall ratio to a 2.35:1.  what does this do for pulling power you ask, well lets say you have a 2.35:1 stall ratio in the converter, that means for every 100 lb/ft of torque coming out of the motor there is 235 lb/ft going into the trans !  Now lets say i decide to go with an allsion 1000 automatic, one of the huge advantages would be the double overdrive.  5th gear in the 1K is .71:1 and 6th is .62:1.  That would get my engine rpms down to where they need to be on the highway for better fuel mileage while keeping my 4:11's in the rear.  Now between the 4:11's and the torque multiplication of the converter the truck should pull off the line like a stripped ape !!  Now, after all this hype there are some downsides to an automatic, the biggest of which is heat.  This can be solved a few different ways and most are really simple.  First of all, and this is pretty basic, get the absolute biggest damn cooler you can fit behind the grill !  Second is as easy as changing fluid in the trans, see, conventional fluids will start to break down and fail at about 230 degrees, this don't leave alot of "buffer zone" when your motor is running 220 degrees and keeping the tranny fluid up in that same area of about 200 degrees under a hard pull.  Now, swapping the fluid out with a good synthetic such as allisons transynd does a couple things.  first and most important is that synthetics don't start to break down of fail until about 300 degrees and if you get the trans that hot you got bigger problems !  The other is that because of the lubricity of the synthetics you can extend your fluid change intervals 3 times which is really good since synthetics can get really expensive.  Now, one other thing to remember is that 90% of heat from your transmission comes from the converter.  So to combat this I think I would go with a "middle of the road" converter (around a 2:1 stall ratio) and modifying the lockup circuit since when a converter is in lockup it isn't "churning" the fluid creating heat.