[Jim Allen]

Let's get a little pedantic just for clarification, shall we?
IMHO, it's a bit of a stretch to say that a free-flow filter will have no positive effect on fuel economy without tests on a lot of different vehicles. It's probably generally true and most true in the modern era where it's easy to compute airflow needs and design a system to match. But if you want to go back to the days of oil bath filters and the first cellulose filters, well, some of those were restrictive enough to be a MPG issue. There are probably some poorly designed intake systems out there these days too but I don't know of any really bad ones. Basically, unless and until you run out of airflow capacity, the intake system is not a negative factor. Considering that hypermiling is all about low revs, even a system that's might be restrictive in a high-revving performance situation has plenty of capacity. A more useful pursuit would be velocity tuning for the most economical rpm range but that's pretty hard for the average person to do