What I hear most often is 80% of the energy consumed by a typical car is used during the manufacturing process.

It's also worth noting that many 80's cars got very good gas mileage. My first car, an '86, got low 30's, and with a tune-up some owners got higher than 35. So, be wary of the studies you read comparing new car mpg to old car mpg. My car got better or equal gas mileage compared to everything but hybrids like the Prius and Insight, and those are not representative of modern cars. I would bet the studies you have been reading have been comparing Insights (70mpg, and actually made since 1999) to 80's Corvettes (14mpg) or something along those lines.