I disagree, that would make the new mustang, new GTO, Camaro, ect a muscle car, when they are clearly pony cars.
American Muscle car:
car
high output motor in performance form
Gearing for accceleration
4 wheels
more then 4 passenger and larger then a pony car
(relitively affordable?)
Pony car:
car
high output motor in performance form
Gearing for accceleration, but also designed with the ability to handle in mind
4 wheels
4 passenger
affordable
Unless you were to argue that the muscle car definition includes the muscle cars of old as well as pony cars, then you would have to include cars like the WRX STI, EVO, ect in the muscle car definition.