How can I improve my gas mileage?

Hello, I'm no expert but I've done a lot of auto restorations, etc. First, I would say you need to measure the outside diameter of the rear tires now and compare them to what they were on the 17's. The actual rim size means nothing in this matter, it's the outside diameter of the tires that matters. If the diameter is now larger you should get better mileage on the highway when you're rolling but accelerating from a stop/start in town will hurt since you're trying to move a larger tire and there is more initial resistance. If there is that much difference in mileage due to the tire size you should be seeing a fair difference in your speedometer reading accuracy. As for engine problems, the sky's the limit without being there to diagnose it. However, if you have a high mileage car you may try having the injection system cleaned at a shop or have the dealer check your fuel flow mapping on their computer system. I don't think it's a fuel filter issue since you are getting more fuel through to the engine now so it's not blocked. Since you have put a more performance type tire on the car is there any chance that you are driving it more aggressively which would definitely increase your fuel consumption and the onboard computer would reset itself for a more fuel rich style of driving? Hope these ideas can help you out. Robi