Dteam270 what motor are you running with that gearing? Some of the old engines weren't near as sensitive to rpm as the newer engines as far as fuel mileage goes.

How about slow down to 65? Or 68 even. Will make more of a difference then running a different tranny or rear gears.

I'd suggest the OP slow down to 60. That thing must pull up hills nice. Narrow gaps between the shifts too.

Not sure if the moter is c-12 or 3406e need to run the numbers on it. For the most part. I do keep my foot out of it trying to get the best milage I can but, I like other people want to maximize fuel economy that sometimes hard when your in a 70mph zone trying to maximise your log book. Just trying to find a balance in it. Looking at the new trucks coming out, if they have 3:55 rears usually they have an OD trans. Well one reason for changing my rears is, one of them is starting to go. So that why I even conciderd changing. If it wasnt for that I would get a OD trans.

correction c-15 not 12

With direct tranny and C15, i suggest you better look at about 3.08 rears!
Cat's do the best on fuel between 12-1400 RPM range!

You have an OD trans.

Its a direct trans. When I got the truck it had a 9 direct in it,but the lo/hi syncronizers were bad. but I drove a year before it finally went out. Never even gave it a second though about going to with a O/D trans. Sure am now. I probably could trade it back in on an O/D trans. Should be able to get some resale out of my trans since its still in good shape. Bought it used at a salvage yard.

You have an OD trans with a .73 final gear.

If you had a direct trans, you'd be spinning 2300 rpms at 65 mph.

What size tires do you have?

Even with the tallest tires, 3.70's with a direct transmission would be 1900 rpms at 65 mph.

I have 22.5 L/P