The good thing about the 3.73 is, direct will still be close to the 4.88 in O/D. So unless you really need the OMG stump pulling 4.88 with Low 1st to get going, the 3.73 is the all around ideal ratio for towing and empty mileage. If you need the "overall" gear reduction of the 4.88, simply drop a gear to direct. 4th I assume with your description of the shifter. A Dana 135! That must be one stout truck!!!
Just for gits and shiggles, you can play with the formula,
mph X rear ratio X trans ratio X 336 / tire diameter = rpm
for any combination you want to figure.

 

I'm glad to hear that you found a complete rear axle, which makes the job a lot easier. Are you happy with the change in RPM's?

 

Unfreaking believable, mpg didn't even change!!!! All this money
money and I'm still getting 9mpg. How come?

 

i'll give you my guess...you previously were reving high=bad fuel economy. now your lugging it maybe having to put your foot down a little more=bad fuel economy. i think you made a big jump 4:88to3:73. i think a 4:30 or 4:10 might have been a better choice. if 4:30 was making your motor rev more than you want you can kill the gear a little with a taller tire. the way you have it now you can only put a shorter tire on to bring the rpm's up.

 

You are absolutely right. 3.73 was what that place had, and I thought it was alright. I am getting 12 mpg of going between 60-65mph, but really really have to watch my foot

 

I wouldn't really go any taller than 4:30s a F-550 empty weighs 9000lbs depending on what is installed on the truck for a body.

 

What I don't understand is with 3.73 gears, 32" tires, and a .72 transmission high gear ratio, the engine should be turning 1,835 RPM's at 65 MPH. Even if he had 3.55 gears, the engine should be turning 1,746 RPM's at 65 MPH. It takes a estimated) 3.25 gear to get the engine to 1,600 RPM's at 65 MPH.

I entered my trucks spec's into the RPM Calculator and it was dead on... I'm confused.

 

I only lost 1.5~~mpg when I went from 3.73 to 4.56 in my previous Hemi powered Dodge 2500HD. It still takes the same hp to move the weight of the truck and wind resistance even if it's at a lower rpm. You should see some improvement but not a whole lot. With a heavy truck, you have to get it up to speed in a reasonable time, not a drag race but not a long drawn out acceleration either. Then use the weight of the truck as momentum as much as you can. Coasting is almost free. Once it becomes habit, it should help. OTR truckers use momentum at every opportunity.

 

Maybe I missed it but is your truck a PSD or V10?

 

It is turning at about 1700 to 1800 rpm at 65 mph, after driving it for 10 hours today I was able to keep an eye on it.

As for coasting it's pretty impossible, the truck decelarates on its own like crazy.

This is 7.3 diesel

 

I wonder if the rapid decel might be a clue. Alignment ok? Wandering or pulling? One thing the SD is not is aerodynamic but I can squeeze some coasting in with my F250. Any extra wind catching protrusions? Wind resistance is a bigger mileage killer than weight. The weight should be no different than an F350 with some weight in the bed. If nothing else, I bet it's a lot quieter at 65mph. My '98 5spd 4.10 Cummins Ram ran 2500rpm at 70. Sounded like a shop vac. My next one was a 3.55 for that reason. Tires aired up hard. Soft tires can eat up 1 mpg. Mine rides like it has wooden wheels but 70psi all around even when empty helps a little. The cooling fan isn't staying engaged is it? That's a good 25hp to turn the big cooling fan. Just thinking out loud...

 

The truck is not always pulling. It's weird I have never seen any other car do anything like that, sometimesit pulls to the right sometimes it's dead on, to point where I can't tell if the alignment is messed up. Once I will get back I will start doing everything to it so it runs great again. I've had I for 3 years now, and honestly haven't done much maintenance to it except what was necessary. It's my work truck, I don't use it much, put only 5k miles on it in 3 years, but this year seems like I will be using it a lot

I just noticed it had 3 dtcs even though CEL wasn't on.

 

My 1998 Mustang GT does that sometimes... I think that it's the crown on the road causing that to occur. At 129K miles, I still have the OEM struts & shocks so that could be the culprit. I know, I should have replaced them a long time ago.

 

i will be putting new tires on it soon, I will get it aligned while I'm at it

 

What were the trouble codes? Anything that might affect mileage?