1fixitman: Dude! I am jealous that you have a 5-speed. The low gears in the 5-speed mean you can run lower ring and pinion ratio gearing in the differentials. As I have stated many times, I really want to change my 3.73s back to 3.55 (or 3.08 or isn't 3.37 the current F150 gearing?) I strongly suggest that you log in and read the stuff written by Dave Whitmer who is the MPG Forum moderator on Powerstrokenation (basically his experience is low RPM means high MPG):

"Gearing does not affect road load, but reduces the engine frictional HP and accessory parasitic load at a given road speed. Engine frictional HP is the power necessary to make the engine turn without making any net power to the flywheel. It is somewhat of a misnomer – most engine bearings are plain bearings which have very low mechanical friction. Most of this HP goes to pumping air through the engine. For International and Cummins engines (and most engines in this class, I’d bet) engine frictional HP goes up proportionally with RPM between 1000 and 2000 RPM and increases with the square of RPM above 2000. Looking at engine frictional HP curves I’ve seen for the International 444 engine, my engine need 12 HP less to make itself run at 1325 RPM than at 2000 RPM. That explains the 3.0 <ACRONYM title="Miles Per Gallon">MPG</ACRONYM> improvement I got over a 3.73 R&P.

Gearing is tough to do. This involves changing the ring-and-pinion set to a numerically lower ratio. Most guys go up in ratio to compensate for big tires but lower ratio R&P sets are rare. I have a 3.08 gear set, but I don’t think they are commercially available anymore. 3.55s can be had, though. I’d expect a 1.0 <ACRONYM title="Miles Per Gallon">MPG</ACRONYM> improvement for a truck that had 3.73s (the most common) and somewhere between 1.5 and 2.5 for a truck with 4.10s. "

PM Dave. He has a 5-speed with 3.08 gearing. It will take a while, but he will get back to you.

My van came with 3.55 gearing which I didn't fully appreciate until I foolishly took the advice of the 4x4 converter to go to 3.73s and I have seen my mileage drop since. According to PMs from Dave the 7.3 has enough torque to power 3.55s no problem and, quite frankly, that was my experience.

I am toying with the idea of doing the ring and pinion change myself as well. I wrote something on that process to Christof13T(?) a while ago. If I can find it I will insert it here. Basically you just need a dial gauge, some bluing, an OEM manual and the patience of a saint. Patience is required because you need to take everything apart to add even a fraction of a mm to shift things. And you use up a non-reusable crush waster every time you do it. But when I took my differential cover off last year I noticed that the differential shop that did my work apparently didn't have that kind of patience because my pinion gear is not running in the center of my ring gear! WTH!!! So, if I have someone else do the work, I will insist on looking at the pinion/ring gear contact before it leaves the shop.

Oh BTW, transmission shops in my experience don't do differentials. They will just add a $1000 for the headaches and sub it out to a true differential shop. Same thing with your local Ford dealer. They will likely not do the work. In Sacramento which is a rather large city, there are only two differential shops that I know of. And a buddy of mine who had a ring and pinion gear changed out at the "best" one has a slight howl. So, go figure.

Nicmike: Please do a write-up with pictures. As far as I can tell from my searches you will be going where no FTE Brother has gone before!