94 Bonneville gas gage screwed up?

I just bought a 94 Bonneville off of my mother so i know the car pretty well. Recently it started having gas mileage problems, so i
took it to the shop for a tune up and a new fuel filter. I checked the fuel tank for leaks and there are none. When I start the car the gas gage goes up and down then goes way down, even after i put gas in it. is it the gas gage if so what do i do?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.