my1st01ss

Any ideas would be greatly appreciated. I have an 01 Camaro SS with approximately 145,000 miles, i just had the tranny rebuilt and before hand noticed my gas mileage was horrible. I thought it was because of the bad tranny, i filled up when i got the car back with new trans and its the same. Its an automatic by the way and im getting 14 miles to a gallon. This cant be right, Its all stock but it was tuned by East Coast Super chargers. My converters are gutted because I replaced them two years ago and they started falling apart and blowing chunks of whatever you call the honeycomb stuff inside them out. My hubby cleared the exhaust and finished clearing them out. I had new plugs installed a couple months ago but not wires. I changed my oil about every 4,000 miles and air filter. I dont know what to have hubby look for? Any ideas would be great....thanks!