how to increase mpg

It sounds like you have one major thing wrong, and one minor. The minor is that the plugs will now be all goofed up. The major is that way to much fuel is getting thru or no where near enough air. The plugs could be the cause and the symptom, but no use changing them until you know or you will have too change them again. You just need to figure out which. Start with stuff that isn't affected by lots of other things, like the air cleaner. make sure you have good clean gas, check plug wires for crossfire, if hard to start warm look to the carb. make sure you are in high gear. Check trans fluid. Feel wheels for dragging brakes. Make sure parking brake is released completely. Use a gauge to determine your vacuum, you could have a leak. Like I said though, with mileage as drastic as that you should be able to isolate the main culprit. I will assume you don't have a dozen big block heads a few brake drums and your mother in law in the trunk.