If you want to be able to top off your 05 - 06 fuel tank you will need a fuel vent mod. (extra 5+ gallons, more accurate MPG calculations)
I installed one on my 06 and it works great.
Do a search for vent kit and you should be able to come up with a link to a place you can buy a kit.
The install is pretty easy. I was a little worried about dropping the fuel tank on a truck with less that 500 miles on it but it was not too difficult.