I would keep the 3.55 for mpg.
Prior to modifications, three weeks after I got my new van with 7.3L, 3.55 diff., stock tire size, factory calibrated speedo, no 4x4, no fiberglass top, etc. I got 21.96 mpg
* driving 45-55 mph on the flats on I-5 from Sacto to L.A.
David Whitmer is the MPG Moderator at PowerstrokeNation.com In his (and my) experience, fewer rpms means more mpg with the 7.3.
See the following three separate threads:
http://powerstrokenation.com/forums/sho ... p?t=119746
http://powerstrokenation.com/forums/sho ... p?t=119745
http://powerstrokenation.com/forums/sho ... p?t=119458
So, speed is not the problem (other than the big box wind resistance), but rpms.
For the same highway speed, a 4.10 differential spins the engine more rpms than a 3.73, and a 3.73 spins more rpms than a 3.55, and a 3.55 spins more rpms than a 3.27, and a 3.27 spins more rpms than a 3.08.
So, everyone who talks about gearing is correct. Mileage can be improved with lower gearing because it reduces engine rpm and engine rpm is what drinks fuel. I have experienced this myself but I try to keep the vehicle as light as possible and tow infrequently and light. If you want to tow heavy or rock crawl, your needs are completely different than mine. A 4.10 will pull a heavy load better with less wear on your engine, transmission and drivetrain, but with resulting lower mpg. That is why F350 duallies ship with 4.10 diffs.
Dave drives a 7.3 2WD with a manual transmission with a 3.08 ratio differential and gets great mpg. So, since my 15-passenger van is intended to haul asses rather than hauling ass, I am going to find the gearing with the fewest rpms with the least torque I can live with.
Which may be back to the 3.55 or 3.27 or even the 3.08 which Whitmer installed in his 7.3."
In a PM Dave said to me:
"3.55s should be A-OK even with that Amish barn you are driving. The 7.3 has plenty of torque at low RPM. I commonly drive in the 1000-1300 RPM range. Watch your tranny. Maybe a tranny temp gauge might be a good idea. Towing and load carrying experience will suffer with lower gearing but mileage will increase."
With respect to Gear Vendors and the toll that lower gearing will take on your engine, transmission and driveline Dave said:
"3.55s might help but you're gonna have to slow down or face frequent automatic transmission failures. A GV would give you the same problem. Slowing down engine speed reduces ATF flow through the cooler and that cooks the tranny. GVs don't work in 4x4 and I've never heard of anybody driving any kind of overdrive in true 4x4 conditions."
4x4 converters want to change the 3.55 differential to the 3.73 or to the 4.10. They do that because the 7.3 has enough torque to rip out your 4R100 transmission (which is the weakest link in our drivetrain) and your 3.55 differential -- if you are stupid and get on it or stupid enough to drive without a tranny temp gauge and an EGT gauge. They also say that a 3.73 or a 4.10 with larger tires gets you basically back to what your stock set up drove like after adding in the extra tire diameter and extra resistance caused by added 4x4 front axle, driveline, hubs, etc.
I call BS on that. Big time.
I functionally "lost" my overdrive when I went from 3.55 to 3.73. Now I can drive up a pretty good grade with my overdrive on. That is crap. Now I get 13 mpg on the flats even driving slowly. That is big time crap.
All because my rpms are up because the 3.73 spins the engine faster than the 3.55 did. And each rpm equals a drink of fuel from each of the 8 injectors.
Here's the deal: Have you ripped up your 4R100 auto trans or your 3.55 diff with your 7.3? If not, you won't with your 4x4 and 3.55 assuming you install a tranny temp gauge.
Here's the deal: You can rock climb if you want with your 3.55 diff. Just put it in 4x4 Low, for crying out loud.
Additionally, I would have your semi-floating axle removed and a full floating F350 axle installed. The FF is wider so you won't need to use spacers on the back wheels which is safer and you can get more weight bearing capacity with the FF and the FF has a NV271 transfer case sensor which your semi-floating does not.
Additionally, and very importantly, I suggest that you immediately drop your steel fuel tank, see if you have delaminating paint in the tank about ready to fowl and ruin your injectors, do the Hutch and Harpoon Mods, replace the tank with a Spectra Premium or a UJoint tank, add a Racor PS 120 3/8" frame rail pre-pump screen filter, and change your oem filter.
* Note this was hand calculated. 7.3 vans did not come with lie-o-meters.
What I have posted above is my personal opinion based on my personal experience.
One of the best slogans I have come across on these threads is Steuber's "Do it nice or do it twice!"
I am in the process of doing mine twice and am a little grumpy about it after laying out $18K for the original 4x4 conversion.
So, do this thread and all of us some justice by posting what you hear in response to each of the points herein.
Better yet, get Stueber to post in response. He has experience with a lot more 4x4 vans than I do. I have only owned two.
We all need to remember that modifications are a compromise. And it is up to the person paying the money to be very clear about what goal they want to accomplish, then the modifier should explain what mods are needed to accomplish the goal and their risks and benefits.
E.g., when your 4R100 goes out (which yours, mine and every other modded and non-modded 7.3 owner's will) and you need a $5,000 BTS or John Woods rebuild, don't blame Chris Steuber.
Hope this helps and Good Luck!
And compliments from one former "dirt bag" to another!