If the muffler is a high flow as well, this is where your mpg loss is. The problem with a completely open exhaust system with Toy engines is the loss of back pressure. These engines need some back pressure to operate at regular driving rpms, otherwise you tend to be deeper into the throttle than with stock systems. This was the case with my 3.0 after installing an open cat and muffler after with headers. I experienced a 2-3 mpg drop, put on a more restrictive 2 chamber turbo muffler and the lost mpg came back. With the turbo muffler I found I didn't have to give it as much gas taking off from a stop, or when cruising down the highway. I would suggest picking up a Raptor turbo muffler from Kragen's($22-24) and see if this changes the loss in mpg, or change to a smaller pipe from your current muffler back.