2.64 with 13 speed double-over & 22.5 LP

I'd say you screwed it up pretty good.

No amount of computer work is going to fix your overgeared differential swap problem.

Go back to what you had and gain back your 33% drop in mileage.

 

No, no, it would probably make it even worse.

Why do you think you can just willy nilly swap gears, transmissions, differentials around just because the parts bolt together.

Multi-Billion dollar corporations (PACCAR, DAIMLER, VOLVO) spend billions of dollars analyzing engine RPM, Gearing ratios, etc.. for better mileage.

Hell, todays trucks have built in software the forward interface radar uses to gauge the hills of the US route you're driving down in Alabama at 0230 am at 65K lbs to tell the truck when to cruise in Eco mode.

Put it back to what it was before, because it don't work now. Then sell the truck and buy one already set up for what you were trying to accomplish.

 

Don't worry about it then, just embrace your new fuel mileage.

Lots of guy on here with largecars, classic Pete's and Kenny's getting 4.5 MPG all day long.

It would be really helpful to everyone if you told us what model year, engine, makes you swapped parts from and to...

EG, I took the diffs from a 1978 International Cabover and put them in my 2013 Coronado, because they bolted together.

 

Computer work will help a little, but your driving will make a bigger difference, get used to not using your overdrives, I run 2.74's and an 18 speed and lo 24.5's and never use my overdrive's when I have a load or any kind of wind is cutting me. I changed from 3.55 and a 15 speed and my ifta everage went from 4.6-4.9 to 6.3 to 6.4 but the biggest thing that helped was learning how to drive the dam thing and not trying to run the rpm so dam low, the only way that works is if the engine was designed to run down that low and even then, they get better mileage but the engines don't last as long because of the heat you generate at low rpm. I never checked the gear ratios on a 13spd but you are probably looking at running a lot in direct and 1st over drive if at speed, I never even use final overdrive when bobtailing it just doesn't want to be there. The other thing you can do is go down in tire size if possible 255 70 22.5 is where I am going next when the 24.5's wear out, if I don't like those I will go to tall 22.5 because they are around the same size I currently run and more widely available then the 24.5's.

 

first off are you taking into account the fact that the miles your truck shows are now way off? get your speedometer set correctly.

second you no longer need to use 12th or 13th gear. heck maybe not even 11th.
getting good mpg isn't just about rpm. it's more about boost pressure and engine load.
keeping those down is usually easier at higher rpm. within reason of course

 

You didn’t lose 33% if your odometer is off by 25% plus .. calculate your mileage by gps until you reset gear ratio value in ecm ..

 

I think you’re getting your mpg from what the truck is reporting or you’re basing it off the mileage the truck reads. I were you I’d fill up those tanks and get something to note down the mile markers you’ve been driving to calculate the true mileage you’re running then next time you top off your tanks note that down you will find out the true mileage. If the truck thinks you’re driving 45mpg when you’re really running 61 then of course the mpg reported by the truck is going to be way off. Heck 77% of 61 is about 47 is about close to what you’re seeing. I do think maybe your fuel mileage dipped just abit but not 33% maybe like 2%.