how much does suspension affect MPG
 

I have adjustable suspension (koni yellows) I was wondering if I sacrifice comfort and place them at max setting will this help?

While I seriously doubt you could see a measurable FE difference, I imagine that softer suspension helps FE -- when you hit a bump, instead of using enough energy (stolen from forward intertia) to push the whole car up, it just pushed the wheel up. OTOH, with harder suspension you may be able to corner faster, carrying more of your energy through a turn.

I would say whichever has your vehicle sitting a bit lower will probably work better.

adjusting his koni's to the maximum level won't directly lower his ride height, but it can indirectly allow him to lower his ride height (assuming he has adjustable coilovers) since the maximum setting will allow less suspension travel.

I plan on lowering my car in the future about .75 of an inch all around (and 20% stiffer springs) to help with aero, handling, and of course looks :D

I say you get the biggest boost from both better handling and a lower stance.

Just make sure you adjust the camber when adjusting the height otherwise you will lose mpg.

and tire life

My Buick areo-modded itself... After 27 years the front springs have gotten a little weak, and the front end is noticably lower to the ground. It still rides nice though, and I'm not complaining about the mileage I'm getting in the car right now. I'm happy with getting over 20 MPG in an antique Buick.

-Jay

The Koni Yellows don't change stroke, the adjustability changes the fluid pass rate. There will be zero ride change regardless of damper setting.
What I didn't know when I bought mine (for the Camel Too) was that they only adjust in re-bound or extension. There is no adjustment for compression rate.
On top of that the extension rate can only be slowed or stiffened from the already sufficiently resistant damping of normal stroking force requirements.
I'd have much preferred that they compression could be stiffer or more resistant than the extension. Whether that is by stiffening the compression or by softening the extension I didn't mind, but I can do neither with the so-called "adjustable" Koni Yellow. I'll not buy another set of too-expensive Yellow again.

i have koni yellows and eibach sportlines on my civic hatch. it made it 1.5" or so lower. i theorized the stiffer springs and dampening help milage as they move less, therefore making less heat, and keep the car in a preferred lower stance instead of boinging around. i'm not sure about any of that, but i am certain that i do not have to apply the brakes on my car nearly as much as i did with the stock suspension set-up on twisty secondary roads or on ramps. i can go a constant 60 (or whatever) with a consistent throttle on mountain roads, having fun, gettin good milage. koni yellows are very highly regarded entry level true performance dampers. gross compression rates need to be controlled via the springs, and the koni's can cope with much higher spring rates on rebound than stock dampers. up to 600 lbs, whereas stock dampers are out of breath after 250lbs. the eibach springs are 350 front, 225 rear (or so, up from 220/120 stock) and i have sway bars front and rear that add spring rate. my set up is designed to yield millage by not having to slow down!!! i guesstimate i gained a mile or two per gallon with my better suspension set-up. i do allot of secondary road driving. results may vary. this is all 96-00 civic #'s.