[Excaliber]

Additional drag with a convertible top down is increased. I recall one of the magazine's ran a top speed test with a new Dodge in the late 60's or early 70's. Convertible, they ran about 150 mph with the top up, less with it down, but not by a lot.

The Le Mans Cobra picked up a few mph with the hardtop, not nearly enough to make a difference. The Ferraris were running the 180 mph range, matching or exceeding that was the target.