[Big Dave]

The substantive mods are the easy part. Its the regulatory compliance that takes so long. Why do you think models stick around so long without more than cosmetic changes.

By time the manufacturers get a high-MPG model through all the approvals, the market can change drastically. If the oil thing really is a bubble (as some people think) and it pops with gas prices going back to 2006 levels, and you haven't got you high-MPG car through the approval process, you are screwed.

I've seen oil price drop off three times since 1974. Oil is a notoriously boo-or-bust business.