If you have stock tires and gears the read out is allmost perfectly accurate. Here is what you do. The only way for it to remain accurate is to reset it EVERY time you refuel. The way to do this is scroll through to the MPG readout and quickly press both buttons. It only takes a couple of seconds and it will flash 0.0 and start recording. It is within a tenth or two every time I do it by hand.

Real good place to get a manual is on ebay. Make sure it is for the year of your truck though. You would be surprised how much they changed from year to year that you don't know until your fixing one.