The lowest elevation, where I live, is about 5,000ft. My car typically runs between 5,000 and 10,000ft. I know that I can run an extra 3psi or so to get it up to stock levels at sea level. I am going to buy the modular kit so that I can change them out easily especially if I go on a trip to a lower elevation. I am thinking that I will get the stock 3.8 pulley for my daily commute as well as the 3.4 for fun on the weekends. Now, the question. I will probably purchase two additional pulleys for when I do the heads and cam. What should I get? I would like one for the heads and cam setup that I will probably do in the next couple of years, and then one when I do a full buid-up later on.

I just filled up my tank last night; forget the mods...with the price of gas, this car is staying stock.

Mark, your mileage will likely go UP as you mod. The engine will run more efficiently. I'm one of the most heavily modded Series 1 L67'* here, and get the best mileage of all of them.

Is it so much that the ENGINE gets more efficient, or that with the extra power you end up being easier on the throttle?

Both, but they're intertwined. Because its more efficient you don't have to press the gas as much. But that doesn't apply to every mod. It takes a certain amount of air and fuel to make the power, it takes a certain amount of power to move the car at 60mph.

Realistically a smaller pulley will lower your mpg though. Suppose you were flowing 300cfm at 2600rpm and that was enough to keep you at 70mph. Well after the pulley maybe you're getting that 300cfm at 2200rpm. You're not as deep into the pedal but 300cfm needs a certain amount of fuel no matter what rpm it happens at.

Mods that increase MPG are things like roller lifters, roller rockers (stock ratio), advancing your timing, upgraded ignitions, or any other thing you can do to make the engine run easier. Just so happens Will has most of that list

JMO though :P

Yes, to some extent...better intake, exhaust (headers), and roller internals will increase mpg. However, a smaller pulley will bump up the boost requiring more fuel as well as requiring a higher octane level. I get away with using mid-grade because of the altitude, and I get between 18-19mpg during stop and go urban driving. Even if I get the same mpg with a smaller pulley while keeping my foot out of the throttle, I would still need to use premium gas resulting in a higher fuel bill each month. I bought this car, in part, because of fuel economy...the rest of my vehicles drink gas.

I am going to do the intake, maybe headers, and leave it. I may get the modular pulley set, and leave it stock for commuting.

Before my pulley swap, with an intake and downpipe, I was getting around 30 MPG on the highway. After I got my pulley swap (3.4") last November, I was getting around 25 to 26 MPG on the highway. It was also very windy out that day, so it may have affected the milage. After that, I got a set of ported manifolds as well as a PCM upgrade, and now I am around 30 MPG on a cold day on the highway.