[MetroMPG]

Buy & read Hucho's book if you're interested in aerodynamics. In it you'll find answers to all your questions, and as a bonus you get to avoid all us rude folks who care more about drag reduction than lift reduction.

Lowering the ride height will typically also reduce drag for a modern passenger car. I recall a chart in Hucho that relates ride height to lift as well, but I've lent my book to Ben so can't look it up. But I suspect it's the positive effect you're after.