I have a 2011 with the 5.0, just changed the exhaust to a magnaflow. Did not touch the cat. My computer if now reading 2 mpg less. I don't now if the loss of back pressure has messed with the reading or is it really getting less mpg now? Should have I left the stock muffler on? And I am driving the same as I always, conservative, since the new trucks have the mpg computer I drive to see how good mpg I can get.

 

I lost mpgs when i redid my exhaust to dual hi flows, x pipe and magnapacks...though I believe the most loss was from the "fun factor"...you sure you aren't maybe goosing it a bit more to hear the happiness?

 

x2 fun factor

 

x3 fun factor

 

Did you remove the resonator and go 3" throughout? Doesn't seem like too much is changing, as the stock muffler and resonator is straight through, but the resonator takes the exhaust down to 2.5" or so, then back to 3" after the muffler. Maybe that made a difference...
I am in the corner of the fun factor, so I'll be no help as far as gas mileage. I just put a K&N on yesterday, and I probably dropped a couple mpg's already (fun) lol. In addition, I just added more rubber on the rims so I'm definitely burnin more gas. I am planning a dyno tune as soon as I everything set this week, so I'll get a good number then... at least a good number for the way I drive!

 

X4....fun factor....

 

as stated above, if you drive the exact same way you did before youll probably get better mpg. dont excpect to be able to see it, or calculate it, but it should be slightley better. but the fun factor of wailing on it will make it worse

 

I like fun factor. Oh yeah, and ...

 

The computer mpg calculator is often referred in this forum as the lie-o-meter. Use it as a general idea if you really want, but trusting it to be accurate is a fatal flaw. The good old pen and paper trick after a fill is the best way. Sometime the old fashioned route is best. With the colder weather here too, a little extra warm up period on the truck really confuses the "lie-o-meter".

 

HAHAHA love that kid on Tosh.0