I have a 98 ford mustang. I have had it for almost two years. I started noticeing the gas milage and power was getting worse each month. Its to the point now I use a quarter of a tank driving 20miles and I have to push the gas peddle down halfway just to really get it to go when stopped at a light or stopsign. Im up to date on oil changes and tuneups and I have also had the gas tank cleaned. What could the problem be?