Yup, the gas gauges bounce around quite a lot. Some of us get used to it and some don't. It bounces a lot because there are no baffles inside of the tank to prevent the fuel from sloshing around and the float just follows the sloshing, so we're kind of stuck.
If you search on "gas gauge bounce" on here you'll probably find that I make up a small electrical circuit that attaches to your gas tank sending unit and it will help to slow down the bouncing. It won't completely cure it, but it does make it a little better. Let me know if you're interested.
Oh, and I'm getting 22-ish mpg mostly highway driving with my 2,110cc engine and I put in whatever 89-91 octane gas I can find. Most everything in New England is E-10 and I'm too lazy to try and find ethanol-free gas near me.