Dodge Charger Daytona and Plymouth Superbird

The Daytona was built as though it was an option package, thanks to a low budget and last-minute modifications to win races - indeed, existing Chargers were converted to become Daytonas. Take a Dodge Charger (440 or Hemi), stick a wedge over its nose and a three foot metal spoiler on the rear, and you've pretty much got a Daytona (this is an exaggeration, as Petty Enterprises' Kurt Romberg pointed out to me. There was also an under-nose spoiler, vertical stabilizers, and a backlight modification, which he notes "are integral parts of the package").
 

The Daytona wouldn't have been complete without at least one quirk, and it picked a good one. Driven too slowly, it could overheat. Increasing speed (or switching to the next-year's Superbird) took care of the problem.
A year later, Plymouth took a Road Runner, and gave it the same treatment. The Superbird was a little slower at top speed, but it had huge cartoon graphics that made 160 mph just a little more fun.
The Daytona and Superbird were considered ugly at the time, but 20 years later, they seem graceful, and maybe even commonplace (except for that spoiler - and their 18 foot length).