Fuel gauge is acting funny

[atoyota4x4]

2000 Intrigue 3.5L

Ok, after owning our car now for a couple of years, I realized this weekend that our gas gauge has been off. I always suspected something but did not put any real thought into it until it started going crazy this weekend. Anyway, we typically always got around 250 miles to a tank, or so we thought. It would only hold about 9 or so gallons when we would fill up. After reading that the tank is a 17gal tank, I now realize the gauge has always been reading wrong and this weekend it must have just totally decided to quit working properly all together. I have read the posts on here from people having the same problem but there are no posts indicating what the end result was to either fixing it or not. Basically the gauge will now not read below 1/2 full and I noticed if you knock it into neutral while driving it jump back and fourth to full and 1/2. Anyway, something is wrong.

One thing I would like to know is since I have never run a full tank of gas through it, what should I expect to get on a full tank of gas? 450 miles? 400?

That way, if I don't fix the gauge or its not worth it money wise, I can at least go by my trip odometer as to when to fill up.

If someone wants to be kind enough to explain the proceedure on fixing the problem, I'm all ears. If I think I can do it, I will