I recently swapped out my stock spark plugs at 91,000 miles on my 98 gt with ngk iridium plugs. $8 each. and put on a set of ford 9mm racing wires, i was hopping to get rid of the pingy noise when accelerating and improve my gas milage. i was getting about 19 mpg before and now im getting 17 mpg. What do i need to do to fix this?