If performance is the criteria, then some 4 door 4cyl cars would be muscle cars. A lot of muscle cars only ran 15's back in the day, and how many 4 door V6's run 14's today?
Exactly. What used to be an acceptable time for a muscle car in the past is now considered pretty slow. But the crop of new muscle is generally much faster than many 4 door V6 cars and thus are a step up in the muscle department.
Note that I am not advocating for the only criteria to be performance, only commenting on the issue of if that is the only factor used to define a “Muscle Car” then someone needs to acknowledge that the new cars have more muscle than the old one when comparing OEM builds.
