Hello. I have a 2006 V6 Mustang.The other day, my tank was half full and I decided to put more gas into it.I put $20 in the tank (a little over 5 gallons).When I left the station, I noticed my full gauge only went from half full to 3/4 full. That much should have filled the car up.And ever since then my gas is going down. I don't want to put more money if its a waste.I never changed the fuel filter and kept up on the oil changes.Anyone know what my problem is and what I can do? Thanks for any advice.