Which 1960s Mattel toy focused on American muscle cars?

In the five decades since they first rolled off the toy line, Hot Wheels has maintained their popularity. In the late 1960s the co-founder of Mattel, Elliot Handler, was looking for a way to improve the diecast car toy market for kids. He felt the toy cars of the day were too realistic to really capture their imaginations, so he hired a designer from General Motors and a former rocket scientist to help him create a diecast car that would be faster, sleeker, and just plain cooler than anything else available at the time.

Source: Johnston Sunrise