How to improve MPG....

Should check CAC for leaks twice a year. You can buy a test kit for $300-$400, or fashion your own for $50 and some time. There is a thread on here (somewhere) on how to make one. You're testing it to see if it holds pressure, about 35-40lbs, over a minute or so.

Look on your specs on the door frame, or glove box, or - look for a metal tag on the diff itself to indicate the rear end ratio.

 

Another thing that at lot do not realise is that new tires hamper your MPG until they get some wear on them

 

Since you have a Cummins here is some reading material for you on how they suggest to increase your fuel mileage lots of good info

https://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

 

ISX with 10 speed and _____ rears on 24.5 talls, you should be running at '____' speed doing '______' rpm, in '__' gear to obtain optimal fuel mileage.

For example, our company has these thing set up like so . . .

ISX with 10 speed OD and 3.42 rears, on 22.5 tall, we are (governed) running at 65 mph doing 1425 rpm in top (OD) gear . . . to obtain optimal fuel mileage.

The scan guage would be a really good tool to show you where the truck will get better efficiency. You can play aroound with speeds/loads/grades/gears etc, and get an idea.

 

First thing is to slow down a bit. 75 is expensive (unless you have a good tail wind) ... going all the way down to 58-60 might be difficult, try doing 65-68 and watch the difference ... it is easier to get down to the 60 mph crowd in small increments rather than going from speedy gonzalez to the tortoise ... and the time difference isn't as much as you would think it is ...

What is the state of your EGR system?

 

Just downloaded and read the entire document. Great stuff. thanks.

 

It can fill up with soot and the valve doesnt function properly. Check rawze's thread. Lots of good info for qll Cummins engines on there.