I installed my lift, CAI, freeflow muffler, 35" tires/wheels all in roughly the same time and noticed my mpg went down considerably. Makes sense, right? Larger tires = heavier weight and revolution times are different. I figured it was the nature of the beast and accepted it.

Well a buddy has a fairly new Rubi. He put a lift and 35s on it. We spoke about gas mileage and he says that his does fine. No problems. Maybe a small drop. I was simply jealous but left it as a fact of life again.

Well this weekend he decided to replace his stock muffler with a Flowmaster 40 series like mine. He just called and let me know that he is immediately down to around 12mpg. Says he has driven about 150 or so miles today and burned a whole tank.

The muffler is his only change and he didn't change location or pipes. He simply replaced stock with FM. Is there a possibility that our JKs require some sort of back pressure to run efficiently and that this could be the cause?

 

maybe a "chip" would tell the computer to adjust for the flowmaster for better performance/mileage?
i did the CAI and notices 1 MPG more instantly.
i started the Borla cat-back today, when i get that on, i have a Unichip already programmed for my mods. i'll post back to see if it gets better MPG or down like you.
i'm no expert, never "chipped" a car b4, just giving my humble opnion

 

the exhaust Shouldnt make that much of a difference, but I have 35's and a lift and I 11-13 MPG. So it sounds like he is getting an accurate reading now....

 

Someone did some dyno tests earlier that showed that the JK likes/needs some back pressure. It is possible that you free flow exhaust is not meeting that need.

 

Different mufflers will dissipate heat differently. The more heat a muffler or exhaust dissipates, the more it reduces exhaust velocity.

There is a sweet spot for a free flowing exhaust and maintaining velocity of exhaust gases. Too much back pressure and you don't push enough exhaust gas from cylinders and too little exhaust velocity and you don't pull enough exhaust gas.

It's not exactly true that a JK likes back pressure, but if you put a 3" pipe on you will have an issue with velocity. Likewise if you put a 2" pipe on you will an issue with back pressure.

 

I can see it. If the muffler frees up more horsepower, you're probably gonna use it and burn more gas.

 

Yup After putting on an aftermarket intake & muffler, my driving habit has changed and my right foot got a little heavier on the skinny pedal... If I drive more conservatively maybe I would see an increase in MPG, but how much fun would that be especially when we drive JeePs...

 

I'd have to check, but I don't think he is the type to change driving habits somuch. Besides - that is a Huge difference. He would have had to be trying out for NASCAR to make a 5-7 mpg difference.

 

i have seen no change in my mpg and i have a moroso spiral flow muffler. if you even want to call it a muffler, they sell it on jegs. sounds amazing best thing i have heard so far. but main thing no change in mpg

 

What are you getting for gas mileage??

I put on 33's and a magnaflow, plus an AEM cia and my mileage went to 12 mpg..!!