gas mileage/cooling problem!! PLEASE HELP

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About a week ago my '97 Pontiac Grand Prix GT began overheating but I was able to fix the problem along with draining and refilling the radiator. Shortly after doing this my gas mileage dropped by A LOT and now I dont know what to do. The fuel pump is working but I went on a trip yesterday and it took TWO tanks to go on what normally takes a little less than ONE. How do I find the problem and save my mileage?! Also, I have two gas gauges - a digital and the standard one in the instrument panel. The standard gauge is no longer working and I dont know how to fix it either. Also, I dont even know if the digital is working. When I'm going on the highway it will display my average mpg and it goes up but the second I start slowing down the mpg display keeps dropping until I start speeding up again. It seems like the faster I go the better the digital display is. PLEASE HELP!