Was getting almost 600 miles to tank of gas. Now getting 250, 300 if lucky. Car runs fine. Whats going on? iPhone ?
MPG is not a measure of Miles Per TANK, but Miles Per Gallon. As bisco indicated, you may have a tank bladder or level sensor issue. There is a DIY fuel level sensor calibration procedure posted on the 'net. Bring a thermos of coffee and a dozen doughnuts, because as I read it (some time ago) it could take a while to complete.
First you need to calculate miles per gallon; miles per tank is a useless figure because there are too many variables.