I'm with Al.... After 20 years, when my tank is "full" the gauge needle stops just below the "4/4" line, like a needle's width.
When my tank is low, the needle bounces around from just below "R" up to about 1/8 tank. That's when I get worried and fill R up. That usually takes 6-1/2 gallons or so to fill and it's a stock 1969 VW tank so whatever they hold, 8-ish gallons? So far (knock wood) I've never run out on the road, even the couple of times it was waaaay down there and hardly bouncing the needle at all - Then I was trying to make a lot of right turns to at least show something on the gauge!
I made up a bunch of gas gauge needle dampeners a while back. They don't completely stop the needle bounce, but they slow it down somewhat - maybe cuts it in half. You can't do much better than that because the gas tanks have no internal baffles to moderate the level for the gauge. I was going to donate a few to the East and West Coast gatherings this year but that didn't happen. I still have a few if anyone is interested. 5 minute installation right on the gas tank sender.
I'll attach the installation instructions for anyone interested.