I recently put the iso thermal plenum on my Gand thought it supposed to get better gas milage but i get way worse and same driving type.. WHy is this anyone have or had this problem could it be something else?

Maybe you're using your newfound horsepower more than you think. I installed one a couple of weeks ago and my milage is the same.

Did you reset the ecu?

What is your driving style and how much MPG were you getting before? If you're able to average 20+mpg, then you're more likely to gain. But if you're averaging 16-18, then it will probably go down with more performance mod.

Just because you drive the same, doesn't mean any product will help or hurt performance. power = burning fuel.

Interesting...

do a ecu reset.. i have the mrev2 and iso thermal spacers too

I have a 1/2" iso thermal spacer i had no problems... But could be because i did the tune right after doing all mods ( i have a cobb tuning access port )

Get a tune. I'm willing to bet you're running rich after the install. Turning back the timing a notch or two might help too.

it could always be the winter fuel

i use to get an average of 22 to 23 miles per gallon now its 19 to 20!!!
I did reset the ecu when i put it on or atleast i think i did it runs fine just the gas milage went down a good bit very noticable....

more air equals more fuel

only thing you have to do is lay off the pedal.

Or there's a leak

Other mods?

NO all stock besides the plenum and no resegnators on the exhaust..
How would there be a leak?

I installed mine a couple weeks and MPG is still the same. I also reset the ECU. I usually get around 16 MPG city driving and about 20 MPG on Freeway.