Several MPG, by my experience.
The factors that affect fuel economy are tire selection, as well as thread width, tire height and combined wheel/tire weight compared to stock.
Two tires, despite even identical size, one an "eco" tire like Michelin Pilot Energy or Continental Eco-Contact compared to a max-performance tire will have a fuel economy difference of several percent. Rolling resistance is the reason, but this is a function primarily of the tire's compound formulation.
Wider tires increase the frontal area, thus aerodynamics are hurt. And contrary to popular thinking, wider tires do NOT necessarily contribute to a greater contact patch or better handling.
Wider and taller tire/wheel combinations have higher polar moment of inertia, and effectively makes the gearing taller (not always a beneficial side-effect). They have higher rolling resistance, and are quite often heavier (even compared to factory steel wheels).
The point is that these have implications on fuel economy (rolling resistance, increased drag), performance (greater inertia and rolling resistance) and handling (unsprung weight). The question is, what are you willing to compromise, and how much?
My personal scenario: A few weeks ago, I replaced the stock steel 6x14 wheels wearing 195/60-14 Badyear Eagle GAs on my Passat for 7x17 aluminum wheels, which at ~20lbs each are considered decently light in the 17" class. With new 215/40-17 tires mounted, I noticed they were immediately HEAVIER than the stock wheels they replaced. The new combo is about 14mm taller (a little more the 1/2") no big deal... actually corrects for conservative speedometer.
I'm sad, though not overly surprised, that my mileage has dropped from mid-to-high-40s MPG to the low-to-mid-40s.
[ September 04, 2001: Message edited by: tdimeister ]