first it was the looks price and OPTIONS , the ford mustang was the first car in the world to offer multiple options such as am or fm radio, leather seats, floor mats, more speakers ect. then the V8's were placed into them and it made it a fun car. Ford offered 2 different V8 engines and a v6 in 1964. Ford sold over 22,000 Mustangs the first day. 100,000 after the first four months. 418,000 Mustangs were sold the first year and the 1,000.000-th in 1966.thats amazing. then carrol shelby got involved. He tuned a 66 mustang fastback and created the gt350, which was raced and won many races. the gt500 was also manufactured. when the muscle car age came along, ford was ready with the gt500 and BOSS 302 and 429 and mach 1. Ford was really thinking when the oil crisis came in 1973 and made the mustang a girl car with weak 6 and 8 cylinder cars. but they out sold everything in its class. The mustang stayed alive by evolving through the years. Today they are awsome to with the gt500 cobra and bullitt and 2010 gt's. but it's the V6 mustamg that sells the most, something chevy and dodge failed to see when their cars were discontinued.