My son has a M Benz 1996 C220 with 135000 miles and he was getting about 400 miles per tank, he keeps a note book in his car so his records are pretty accurate. He is down more than 25% on MPG's. He took the car in to a shop and they ran emissions check, changed air clearner, added fuel cleaner and inspected for leaks. He has a little more power but he is still way down on MPG's. Any ideas?