I had a pair of Flowmaster mufflers installed on my '98 Mustang. Ever since then, my mileage has been down about 10%. This has been nearly three months, so it's not based on one tankful. I have reset the computer by disconnecting the battery... since that time I've noticed a change in temperment, but the mileage is still low. My theory... flowmaster makes the claim that their mufflers send so much heat out the tailpipe that engine components such as belts and hoses will last longer. Well, I don't know about that, but I'm wondering if there has been a change in temperature at the O2 sensors to the extent that this could cause a drop in mileage? Maybe, (hopefully) this would be happening only because the sensors were marginal to begin with. No other exhaust system mods, the sensors are original (I believe) with 80K miles on them. Does my theory make sense? I also thought it could be "winter gas", but last winter my mileage wasn't this bad.