If the truck is stock, it should do better than that. 13-15 would be a minimum. Don't do gearing as stated above. You have another problem, which may be related to fuel or breathing. On the breathing side, check your EGR valves. They are notorious for getting sooted up and if they get stuck open, you'll be running a low-oxygen mix all of the time, not just low RPM where it is intended to operate. You can remove it and clean it, or if it is really bad, replace it. Check the air filters too. The stock unit is very good, which means that if it is subjected to low quality air, it could be clogged up. Clean or replace them. One last area to check are your air sensors. If they are not good, then your programming (stock) won't be efficiently sending fuel (air sensors are part of the equation). You'd have to get FMC to run some diagnostics; I would do this last if the other stuff does not help.
On the fuel side, it is a combination of the HPOP and the injectors. I agree with the above to check the fuel filter or replace it. The injectors should have been resolved by 06' but you never know. Get a bad spool valve or two and you could be weeping, and that would absolutely kill mileage. If the HPOP is not all it used to be (not so likely, but it could be a problem), you won't get the atomization needed for optimum burn. As with the sensors, do this last.
So, start with the EGR and the air filter and fuel filter. If none of that helps, have FMC check the injectors, air sensors and HPOP.
Keep us posted, Ralph
