I just bought a new 70 series flowmaster for my truck. I didn't do anything special, just dropped the old muffler, and stuck in the new one. All factory piping still being used, just a muffler swap. According to the computer display, my gas mileage went down. I was expecting it to go up. My question, does the ECM need a bit of time to adjust, or is it something else. I'm a bit ticked off at this point because I was expecting some better mileage. I gained about 1.5 to 2 MPG with a K&N cold air kit, now I seemed to have lost it. Surely I didn't open it up too much.


BTW....I'm not driving around goosing the engine cuz I have a new cool sound, the 70 series is rather quiet.

 

That topic has been discussed a few times and it seems it depends on the truck. Ive heard guys say they gained 1.5 to 2mpg and Ive heard guys say they lost 1.5 to 2mpg. For me when I added my flowmaster I gained a little but I went from a single in single out muffler to a single in dual out muffler. Its hard to say why yours dropped or what will make you gain it back!

 

I didn't gain or lose any mpg with the muffler. It went up with the diablo tune. I installed it in January and I went from 15.3 to 17.7.

 

Maybe where he still has single in, single out, but i'd have thought MPG would have atleast stayed the same...... I can't get a true reading coz i like the sound of my 50 series straight out the back. But i guesstimate about 15 around town. I live on outskirts of town, its only3 miles to work, and during week, i generally stay close to home. Average fuel consumption from Thursday night to Thursday night is about 1/2 tank. And we use my truck on weekends as well as my wife travels 55 miles round trip back n forth to work Monday thru Friday with her '02 Avalanche (5.3L). She uses a 1/2 tank as well a week.

 

It is odd you lost that much even retaining the stock piping. I lost about that much when I had my Flowmaster 40 installed but that is a bit more drastic than your muffler and it's also a dual outlet muffler so I lost a lot more backpressure than you probably lost.

If fuel economy is your concern I'd wait a couple days and see if it picks back up, if not, plan on putting the factory muffler back on I guess.

 

After a few days, it seems as though something was wrong with the first tank of gas with the new muffler. So right now I'm geting about 16.6 MPG to this point. I still want better milage, I think at this point I'll get 18 on the interstate. What will get me better MPG's? programmer, headers, high flow cat,....?

 

Predator programmer with 87 octane tune using the canned tune you gain 1 mpg overall.

Use the 93 tune and get 2 mpg gain.

Dial it in by adjusting the Long Term Fuel Trims and Timing and you can squeeze another 1 mpg out of either one and gain a couple of extra horsepower.

So...

93 Dialed in you can get about 3mpg gain. I did this and get roughly 22 mpg on the highway and my truck is a 4x4 and PA is hilly. (But I drive conservatively) But either way I heard that the "dial in" can overcome the mpg loss due to aftermarket exhaust so even with an 87 tune dialed in you could get the 1.5 mpg back and maybe gain another 1 mpg while increaseing output roughly 7 to 10 hp. (93 would be 10-15)

The biggest impact on mpg with these trucks is the shifting, if you can "dail in" the shift points with the predator to your driving style and conditions you can further improve mpg.

I am not all nutzoid over the Predator tho, I mean you can easily go get a dyno tune and get the same results but I would NOT go with a canned tune or canned programmer tune alone.

 

you cant put a bad *** muffler on and expect to get better MPG. I'll bet you love that sound so much that you cant keep your foot out of it. Thats all it is. I guess i never told you guys that my job gives me all the gas i can handle, in fact i have an aftermarket 45 gallon fuel tank under the truck and a 90 GAL AUX tank in the bed withA pump on the tank to fill up all my other cars. Sry for bragging.

 

Maybe if you'll read the whole post, you'll see that I have a 70 series muffler....not loud, and you would also read that I'm not putting my foot in it. Why would I when I'm trying to SAVE fuel.....

 

Okay, went back to the muffler shop and they did some checking and supposedly the new muffler F'd up the old cat's. There was 6 and 8 pounds of back pressure at the cats when there should only be 2 or so, and no back pressure at the muffler. The only thing is, they didn't do that same test after the cat swap. There was 80k miles on the oldcats anyway. So I put new cats on it, and while I was at it I went ahead andput high flow's in. Now I regained the power I seemed to have lost after the muffler add on, but I still am no better off with feul milage than before the muffler. What the hell have I gat to do here, any ideas?