Why is my fuel milage so bad?
 

I am only getting around 14-15 in my truck. I am going to change the engine oil and I want to change the differential and transmission oil but I need a new job first. Anyways, is there maybe some other reason that Im getting bad milage? I put a new CP3 which was what my check engine light was on for and my diesel mechanic said it should fix my problem if I got a new one but even after I replaced it the check engine light is still on. I took it to autozone and they ran the scan tool. It said the truck passed but the light is still on. Do I need to run a different kind of scan tool on it? What could be my problem? Thanks guys.

What was wrong with the CP3 that it took a crap? Whens the last time you changed the fuel filter or clean the air filter? Are you runnin any type of programer/chip? Auto or man? Are you hand calculating the milage or lookin at the overhead monitor? Also is the speedo recalibrated for the larger tires? Lotsa possiblities... :pca1:

Oh boy! The CP3 failed because the lift pump failed and the guy that owned it before never fixed it. I changed the fuel filter less than a month ago. Cleaned the air filter when I changed the fuel filter. No chip. Manual. I am hand calcualting and the overhead says im getting 3 miles to the gallon better than what im calculating. I don't think the spedo has been recalibrated. I also dont know what kind of gears I have. I would think they are 3.73s but maybe they are something else.

It should have 3:73 which is what mine came with and I know 35's threw off my speedo by 5 mph verified by gps. That will def. screw with you mpg readings! :pca1:

What should I do then? Are the 3.73 gears ok with 35s or do I need to put 4.10s or something?

The gears are fine, you just need to get your speedo recalibrated. Dealer might be able to help, mine wouldn't. I just reprogramed mine with my Smarty. Because with the speedo being 5mph off the trucks mileage is showing less than you actually traveled hence bad fuel mileage calculations.

Thats where im having a lot of problems. I don't have access to a smarty anywhere and the dealerships around here are only out to take my money. I will see if I can get a hold of one somewhere along the line. Thanks for all the help!:U:

I wish I were closer.. I have a Smarty!
I feel your pain... I have the exact same problems with mine! If one of us ever figures out what it is we will have to make sure to tell the other!
Look in your glove box for a label that has your gear ratio on it. Or you can go here and see where your rpm is on the chart for the equipment your truck has. You can even change tire size to a different value to re-figure the rpm!

klenger.net

To be honest the only thing I haven't checked on mine is the intake system for leaks (Under boost). I started buying the 'tools' to check it and never finished. Guess I better get that done.
Let me know if you find anything or if you have more questions.
HTH,

(Check around, you may find a dealer that will do adjust the speedo for you! There are some good ones still out there!)

some of our more "advanced" genral garages in our area are able to refigure for larger tires. i put a little bigger tires then stock on my new chevy, and the garage that did it, changed it right away, with out even askin me. and he was some backyard mechanic, with a really big barn for a shop.

Try cleaning the AIT (air intake temp) and MAP (manifold absolute pressure) sensors, located on the driver's side of the engine in the intake manifold above the fuel filter. These can get carboned-up and throw off the readings to the ECM, causing it to dump way more fuel than needed.