Ok I have a stock 2001 2500 with my stock exhaust, with a full tank it lasted over a week, now I put a straight pipe single stack. 4" straight pipe to single 5" I dont know what happend but I get horrible milage If i get half a tank it is gone about 2 days. what is the problem, i havent done anything exept the exhaust.

 

First off the top of the tank is bigger than the bottom. When I fill up it stays on full for 80 or so miles. Second are you on the throttle more so you can hear your new stack?

 

Thats what I was wondering... my milage for the first couple tanks after the stacks went on was terrible! I just love the sound of stacks on a Cummins!

 

1/2 to Full in regards to the tank is larger than 1/2 to empty as xzaq103 said, so that could be throwing you off as well. The only thing that is robbing fuel mileage if you only changed the stock exhaust to a stack would be you being on the skinny pedal. Your ears can't get enough of that sound so you are always on it

 

I've got a straight piped exhaust... And I've got awesome 21.7 to 22.5 MPG almost constantly...

So I think its got a lot to do with you foot on the throttle and the speeds your ripping down the road... My average speed is about 40-45 MPH on a full trip (cruise set for 55 MPH typically)

Keep you boost below 5 PSI and Pyro under 600*F and 20+ MPG will always happen... Like in my signature...

My MPG at 65 MPH...


My MPG at 55 MPH...


And for my ScanGauge II is calibrated and very accurate... I can run my tank down to 2-3 gallons and not run out.. The amount of fuel shown in the amount of fuel I pump into the tank to fill up!