Well, I'll weigh in on this one since iz not just a hillbilly but a motor head hillbilly. The big three US are always designing and building prototypes most of which never are produced for sale. Now, if you'll remember, GM started the Saturn Division a few years back. The Saturn concept was to kind of start from scratch using GM subcontractors but allowing an indepenent group of designers, engineers, body molders, etc. making Saturn not just another GM model rolling off the same old GM lines but a hybrid. The Saturn program took 3 years. The new Beetle, in spite of being bult on an existing Golf chasis was a 4 to 5 year program. Part of this was marketing and part was engineering and production. Some of the changes in models occur because of safety and emmissions regulations which force the manufacturers to bump up their time tables. One last car that I know about is the Mustang whoes prottype had been in the box for a couple of years when Lee Iacokka pushed Ford to produce it. Had he not been so pursuasive, the Mustang might still be in the box... So, there more to the answer than just thbe engineering and mechanics. Marketing is a major variable.

Bobert