When I first got the truck, (my first diesel) it was easy to figure out the gas tank register and MPG was off by ten to even 20 percent. I let it go and would fill it based on the needle on the gas meter. I should've been able to get 30 gallons in it, but as some have stated, it was never more than 24 to 25 gallons.
A few months later, I was pushing miles and time on the highway through Nebraska, gauges said I had 50 miles of fuel left and next fuel stop was 22 miles out. Easy Peezy. But that Fuel stop was shuttered and closed. Next stop was 20 more miles, but at 3 am, it too was closed. I was getting a little nervous about being stranded on a highway in the middle of Nebraska, google-mapping for a place for fuel up ahead. They were out of diesel!
Continuing onward, with the needle buried into the letter E and the light glaring at me ... I made it to another station and filled up. I put only 28 gallons in it. While there I ended up talking to an old timer pumping off the other side of the pump. He told me how bad it was to run the tank that dry, especially on a diesel (I shamefully admitted I knew not to go that low), and then he told me to "Rock the truck violently from side to side". This will "Burp" the tank and allow for more fuel. It's not something to do too often, but it will help with setting the gas meter better.
I kinda laughed at that thinking its some ancient trick for cars back in his youth. But he came over and started rocking the truck. After about 10 good shakes, he told me to add more fuel. And I was able to get 4 more gallons in totaling 33 gallons.
Since then, my gas meter and range estimate are a pretty good 'guesstimate' of how much I've used and what's left.
F'ing old timers ... they got a couple good tricks.
I don't know if this will help any of you ... but I hope it does.