Can restricting airflow actually increase mpg

[Christ]

The air filter thing is a problem on Carb'd cars, worth noting.

On a carb, a combination of vacuum and flow/mass suck fuel from the jets into the air stream. However, much like the way a choke operates, forcing the engine under more vacuum (dirty filter) will draw in more fuel and make the engine run rich. Sometimes, VERY rich.

This doesn't happen with the throttle plate because the throttle plate is below the jets. Since it "piles" the air mass in front of the plate, around the jets, fuel essentially stops flowing (slows down so much that the engine will only get slightly more fuel than idle requirement).