A kid around here autox’d a HF once. It ran the entire course in first gear, and he said it only needed two gears to clear the quarter mile haha. But I’ve heard all kinds of weird stories behind the HF’s. Thinner window glass to save weight, either the body or just the fenders were constructed from a slightly thinner gauge of metal, I think they lacked some sound deading, etc. But yea either way, they could pull like 40+mpg back in the late 80’s.