Before I went thru changing rears, turbo, etc I would put 24.5's on the truck. I had a 96 Pete, 550 CAT, 15 speed, 3:55 rears. Truck was on lo-pros and turning 1700 RPMs at 65 MPH. Hell with that! Put 24.5's on it,dropped the RPMs to 1450 # 65MPH and averaged 5.93 MPG for ALL miles. That ain't bad for a 379ExHd with a 550CAT. Dropping those RPM's to a more manageable range helped more than anything I think. I pulled a reefer and pretty much ran loaded at 79K+ and I wasn't about to pay the $ for new rears etc. I did keep the trailer as close to the cab as I could. Loaded 45k in the trailer, set the 5th wheel for 12k on the steer and went about my business. Just a suggestion.

You can do the fuel harness pretty easy and cheap. There is a plug on the top of the fuel block behind the primer pump. Unplug it and look at the terminal size of the plug. Go to Radio shack and get some pins that are the correct size and get a 1 watt 150 ohm resistor and some wire. Solder some wire to each pin then solder the resistor to the wires and heat shrink everything up nice. Install your new harness into the plug that is behind the fuel primer pump. Tape everything up to seal it up and you are done. Total cost would be about 5 bucks. It might get you a couple tenths but that is it.

Spend some money and get the truck and trailer totally aligned. Get them done while hooked if possible. I did this with my own truck and milage went up a couple tenths. I had had both done seperately but did not see the results that I got after doing them together.

Get rid of the things that are not absolutly needed in the truck. Less weight to carry around.

Oh and to mimic what another member said. Get the trailer up as tight as possible. I am two notches from being all the way forward on my 379 and my weight seems almost perfect when loaded.

I have seen some skirts on company trailers that look interesting. They have got to help some as they reduce wind drag under the trailer.

Keep it below 68 mph when ever possible. You will only loose maybe 15 mins of driving time at the end of the day. That is less then 30 miles. Don't lug a bridged motor. Let it rev when needed. I rev my c15 bridged to 1700-1800 on hills etc and have seen a slight improvement in fuel economy.

But be happy. I run an 04 379 with almost the same exact specs and the best I have gotten is 5.0 mpg pulling a reefer at 78,000 gross. When hooked to my belt bottom Eagle bridge grossing 98,000+ I average about 3.2.

fuel wire will help i have tried it, the boost wire that only improves spool up and gives u smoke, i haul up to 120,000lbs and i wont consider lugging it i keep it over 1500 at all times on the grades, lugging only sends up temps, see how i load sand in the pic below

__________________
"load em like rail cars drive em like race cars "

not 100% sure mine can be boosted to 500hp, that would be done in the ecm correct? something cat would have to do.
is yours also a bridge? mbn ?

question, you say 5.93 on all miles, is that before or after? did you gain any mpg by swapping from 22.5 to 24.5?
im not sure on tires and stuff but i understand the rpms would go down, but you would also have more to turn, there is more resistance on a 24.5 then a 22.5, but more rpms on 22.5 then 24.5's.

i probably got a slot or 2 i can slide the trailer closer to the cab.

now on the fuel harness, from my understanding of reading last night was that the fuel harness trick will only work for a short time, when its cold, so after the fuel heats up in the tanks the fuel harness isnt even working, and in the hot of summer it must not work at all, i think i read where once the fuel reaches 150ish it dont matter no more. if that is true i cant see it really being worth it, i could be wrong but trying to read up on all i can here and tis a bit overwhelming and confusing.

on that note, if i was to do this fuel harnes, is there ANY way to see its working once its hooked up?? or if the truck runs its done right?

got another question, read that if i change my ecm to a 6nz ecm that will help "un-bridge" my bridge motor, is that true? what will that exactly do?

would it be good to get that ecm, and a good full flow muffler and a better turbo?

thanks for the help guys.

Lots of VERY good information can be found here, just have to do some searches, and then make up your mind how far you want to go with the mods.

As an owner of a 2004 MBN, (as in Much Bad News), I did a lot of experimenting, and warn you:
(1) STAY AWAY from the turbo boost wire if you have a stock ECM. it will throw a code, and if you do not remove it, it will de-rate the hp. (mine did anyway)
(2) The fuel wire made no difference on my truck. I thought my resistor was bad, so I swapped with a friend who had one on a 6NZ; both of us got no change, so we swapped back. A fuel cooler would probably help me more here in south Lousy-ana's warm weather.

I pull a tank, licensed for the 48. When loaded, they try to keep us at 78,500, and deadhead back for about 95% of our loads; (specialized products).
When I purchased this truck used, ECM MPG was 4.86. (W900, 475, 13sp, 3.36, tall 24.50. My biggest gains in MPG were when I changed injector codes to 5232, and replaced the catalytic mufflers with the regular style under sleeper mufflers. Began averaging around 5.3.
Because of numerous engine problems with this truck, I made some improvements while making repairs- so kind of hard to say how much any single mod would help you, unless you had the same problems as I.
Due to flaking, replaced stock cam with the 3406E marine, then had to have my ECM, reflashed to 550 6NZ, (Thanks again, DDS), so the cam would work. This was a tremendous improvement, both power, and MPG.
Shortly after, I had to have the engine inframed, and replaced a faulty turbo, (washed divider plate), with the Borg Warner unit. (approx. 8 more #s of boost, and cooler egts with the new turbo)

Did a couple of other small mods, but the above stuff is what made the most difference for me. MPG is consistently in the high 5s, and low (I mean "real low") 6s.

If you are serious about improving your mileage, you would benefit by doing a search of my posts in this section; I did a lot of crying, complaining, and asking questions trying to improve my MBN. There are some really helpful guys here that have a lot of knowledge to share.

READ, ASK, and LEARN!!!!!!
Good Luck!

has anyone with the same motor as me swapped ecms and seen a good difference in mpg/power?
cat c-15 435hp serial mbn (bridge)

if i get a 6nz ecm, do i need to change ANY thing else to match it?
if i get a 6nz ecm, should i change anything to compliment the change that will benefit me more?

im also really curious on the rear end gear changing, if i get my rear ends changed out for approx 2k and it drops my rpms in 12th by 1-2k, in theory i should be gettin the rpms i want at 65ish mph.

im not doing anything very soon, trying to build a plan and figure out what exactly is the best thing to do.

thanks

435hp motor tall gears and a 75k gross, I don't think you will get results you are looking for, your rpms are about right for the hp you have