I just had an 18" magnaflow put on my truck. Small but noticeable increase in power and around 0.7-1.0 mpg increase (during the week I had it). I wanted better sound so I had the muffler shop put on a flowmaster 40 series muffler. I have only driven it for 5-7 miles, but it seems like I've lost power compared to the magnaflow, possibly even from stock!! I'll test it out a bit more this week, but has anyone else noticed no power or power loss from a flowmaster 40?