exactly... Every mod I ever did on my LT1 cut deeper into my MPG..
and when we talk MPG, I have to say that I probably am the most detailed on this board..
My LS1 is bone stock and does as many miles as you are wanting to do on average..

http://www.pizzaplacetx.com/2002mpg.html

 

rule of thumb is anything that improves your induction, exhaust, and spark will give you better MPG. However, this does not apply to forced induction.


So a lid, a tune up with better plugs, exhaust (never heard of LT's giving bad gas mileage?) LT's, Catback, Highflows or ORY. should all help out of gas mileage to a point. Toss in a radical cam, and head job then yes you will lose some mileage.

 

i put and slp intake lid, k&n filter, and a smooth air bellow, and bassani exhaust, and as long as i dont drive hard and just cruise around the speed limit everywhere i get about 19-21mpg. That is good enough gas milage for me and i spent about 1500 dollars on that.... just a thought

 

here...let me put this super/turbo thing to rest...

KEEPING YOUR FOOT OUT OF IT, a supercharger will still decrease mileage more than a turbo. Why? It does not matter ANYTHING where in the powerband a supercharger or turbo kicks in - roots superchargers kick in early, centrifugal superchargers kick in higher up in the powerband. This doesn't affect what kind of gas mileage you'll get though. A supercharger is belt drive, thusly causing parasitic power loss. Yes, it does produce a good bit more power than it takes to make that power, BUT being run on the accesory belt, it is "always on". Constly pumping air. Maybe not so much compressed if your foot is out of it, BUT...it's always on.

A turbocharger, on the other hand, is exhaust gas driven. Again, smaller turbos kick in lower end, big turbos wait till higher in the RPM range to spool up. However, turbos are NOT always on - keep your foot out of it, and you'll never see boost. It's not belt drive, so it's not causing a power loss. It's just sitting there, so mileage is not affected unless you go into boost.

 

that would be what i said.