ok so I have a 6.9 turbo with a zf5 and 4.10s. I was going to drop the 4.10s and pit in 3.55s but I still don't think that will give me what I need. I need to be able to pull big loads and maintain low rpms at fairly high speeds. Time is money. So I was thinking keep the 4.10s and get a overdrive unit. But I don't want to pay a grand to accomplish this. Gear vendors are pricey so I was thinking spicer gear splitter "brownie box". But were can I get one and what are other options. Right now 65 mph is at like 2600rpms I would like to get 70mph at like 18-1900 ish rpms. HELP!

 

I used to have a 86 with a 6.9 and zf5 and 3.55's got around 28 mpg out of it, would pull heavy loads pretty well as long as you kept it in 4th while pulling, if i remember right 60 mph was about 18 ro 1900 rpm.

 

Well that's not bad. There are two zf5 that came in fords and I have the second one. The powerstroke one. I think mine has a shorter OD so you can tow but I can't remember. Know where I can get like a 1:1 plus 28% OD unit from. Nothing fancy cuz if I could split 4 with the 28% then shift into 5 and on flat use the od in 5 as well I'd be good