I'm a new Civic owner (97 LX, 5spd, non vtec, 188K ODO) and bought the car for commuting 60 mi. daily so MPG is important to me. The car already had an intake, 2 1/4" axle back and a loud stainless muff that just had to go. With the loud tip I was recording 28-29 mpg and since going back to a stock muffler I'm now recording 33-34 mpg mostly highway. I'm in shock, 5 mpg increase just goin back to the stock muffler? Someone throw me a bone here....................

 

hhmmmm... sounds odd to me, Usually the Exhaust/Intake upgrades UP the mpg, not drop it.

 

did the temperatures suddenly change?