Its your flowmasers, over and over it has been proven that flowmasters rob power.
All my mods have assisted my truck to getting 18mpg city
Magnaflow catback
pace edwards bed cover
lighter wheels/tire combo
airraid intake

You have a few things working against you, 34" tires are heavier and change your speedo its a 12% difference. if you want to keep your tires you could swap in some 4.10's or better which will also help your mileage

intake and exhaust will help, more efficient pump=better fuel economy

check with the sponsors for a tune Wheatly or nelson, both great tuners. you will get more power and efficiency with their tunes

 

So you my flowmaster is robing power? Maybe I should get a magnaflow, but they have always been a bit quite at idle for my taste. You said its been proven, proven by who?

 

Go over to ls1tech and do a search, I saw atleast three different dynos showing over 20hp with cutout open, with flowmasters. with borla and magnaflow it was a hit or miss gain.
you can see a straight 3" hole through my muffler

 

https://ls1tech.com/forums/dynamomet...they-suck.html

 

now if you have the race mufflers thats different but if its their classic chambered style it will sound good but not flow good.
on my camaro with the cutout open it was a whole nother animal up top.

 

Sorry not trying to hijack this thread, but I have the super 44. Really, I don't know what the internal design of it is.

Back when I bought this muffler I didn't really know a lot about exhaust performance and just based my decision on the sound, and btw I think it sounds great especially with the new LTs.

 

i did the same when i bought my camaro then started searching around on ls1tech.
magnaflow is not the only option. spintech, borla, corsa all receive great reviews and they have straight through designs also

 

I heard a set of spintechs on a GTO and they sounded pretty sweet.

 

Ive got a CAI, P/P TB, full exhaust, and 1.85 rockers, and I got 24.1mpg on the last highway trip, but I have 265s and 3.73s...

Id say youll notice a couple mpgs less with the tires, 1 because mickeys are heavy, two because not only are the bigger, but they are wider, which means more rolling resistance as well.

 

thanks for all the input guys and don't worry about stealing the thread. the more people type the more i learn too.

Yeah, i'm just going to have to suffer with some loss because of the tires because i like the look of the wide tire sticking out and i've never had a bit of trouble with MT's and these ATZ's are unbelievably quiet and smooth.

I think i'm going to get a get a tune, CAI, and full exhaust all at the same time. Now to decide which exhaust i want to go with. LT's and Y? Mid's and H-pipie? Decisions, decisions...