[redpoint5]

The 2003-2004 Ram/Cummins is my dream truck. Did the 2004.5 add a cat? I can't remember what year added it.

1. I believe my final drive ratio was found somewhere under the hood. In a manual transmission, speed is directly proportional to RPM, so we should be able to determine your ratio if you post what speed/RPM you are at in top gear.

I read somewhere that my truck cruises most efficiently at around 1600 RPM. With my oversized tires, tall final drive, and automatic, that's about 55 MPH.

2. The general rule of thumb for air dam is to have it hang down to the same level as the lowest component on the vehicle. That would probably be the rear differential.

I'm no expert on any of your questions though.