having problems with gas gauge

I am wondering if anyone else is having problems with there reading on the gas gauge. I can never know when to fill it up ot if it is full.

I have only quirk with my 94 blzr gag guage. I get about 300miles out of a tank. but this is how the guage reads out in accordance with mileage. miles are approx.
3/4 - 130 Miles driven
1/2 - 200 Miles driven
1/4 - 255 Miles driven

it seems that the fir 200 miles the guage goes down very slow, then once i hit 200 miles / half a tank (according to guage) the guage flys down.

My guage doesn't even work. Most of the time it says the tank is full even when its actually empty. no idea how to fix it.