You're right...some gauges don't go to E with the key off. Generally speaking those are the gauges like the one in your 57 (which sits horizontal). I'm working from memory (and it has been a while), but I believe it is perfectly normal for your gauge to stop where it was when the ignition was turned off.
So I decided to drive and add some gas to see if anything changes and so I added 5.5 gal and the gauge moved half between 1/2 & 3/4 which by all calculations is about correct. But when the key is in off position it holds where it is now. So I have to believe that on this truck the gauge holds to what’s in the tank and doesn’t return to empty. So for now on to the next item on the list
