I see some trucks that have the over head computer that shows the 'mpg' mine dont have this but I was thinking maybee I can get one from the junk yard and put it in or do I need a new ecm and a bunch of wires or is it all there ?

 

You'd probably have to cut into the steel in the roof,and I doubt they just ran extra wires and connections up there for a vehicle that was unequipped from the factory. Also you'd probably have to install all the sensors, compass, etc.that the computer uses to display data.I'm not saying you couldn't do it, but it's a lot of work for the little the computer actually does, plus they make aftermarketGPS devices that you can just mount to the dash that would do all the computer does and more.

 

Sometimes the wires needed in the harness are already there
but you do still need a PCM computer with the right programming
to successfully put the overhead trip computer in a 2nd Gen Ram
that didn't have one from the factory.

The Scangauge OBD-II diagnostic scanner display is one option to easily add a trip computer.

http://www.scangauge.com/

The Oztrip computer kit or assembled gauge from Australia is another option.