According to most sites, the spec is 14.5 gallons on a tank. I think if you're getting more than 15 gallons in the tank, you should report that gas station because you're probably getting ripped off.
There have been a couple of times where I was at a cheap gas station where I thought I was putting in a little too much gas because I hadn't thought the gas gauge was reading that low, but I've never really put more than 14 gallons in. I think it's also bad to run your tank so low that you need it fill it all the way up, I'm told that it burns out your fuel pump prematurely as the gas is used as a heat sink to cool the pump.
Oh, and I've always gotten around 20 mpg, sometimes 22 mpg when there's a lot of highway.
