My gas gauge tells me that I have 347 miles to E, then the next time that I get into my truck it tells me that I have 300 miles to E.
What is causing that?

 

No way can a computer predict that with any degree of accuracy. That's just the way it is.

 

Do you park your truck on level ground? I notice on mine, that if it's parked nose up on a driveway, the gas gauge goes up. If I park nose down, the gas gauge goes down, until it all levels out again, then it's back to where it really is.

I have the 36 gallon tank, so if I am down to a 1/4 tank on the level, when I park nose down, the low fuel light will come on..

Even driving up and down the grades in the mountains will cause it to change if it's a long grade..

It's just a 'best guess' and most, so don't take it as Gospel..

Good luck!

Mitch