Gas gage shows full then all over the place?

you have a few options on this.normally when this happens,it's the fuel level sensor(float/arm/switch)$65.00 that has gone bad,and NORMALLY you could and still can just replace that part only,or most will suggest replacing the whole fuel pump module assembly around $300.00,sensor included.the last option is just live with it!reason being on this year Buick and more years,i have read and seen where folks have spent up to $1000.00 or more(at the dealer) having this done,and it works for a while,then starts the same crap again later.some say they turn the key to on,before starting and the reading is correct,then once started and driving,the fuel hand is all over the place again,but they have a good idea of how much fuel they have before getting more.or,you could do like some and use the trip button on the odometer to keep up with around how many miles you get to a full tank of fuel.ok,look to the right of this page,click on problems(16)for your car,then click on "Erratic Fuel Gauge Operation" and then start reading.if you decide to change the pump or the level sensor,the good news is that there is a fuel pump access panel in the trunk,behind the back seat,under the carpet.so you don't have to drop the tank to replace it!!read,see what you think!