Car sales at dealerships are revving up as price-conscious consumers trade in their older car, for a new or slightly used model. Upcoming earnings from four top owners of auto dealerships should offer insights into the road ahead for vehicle sales. The average age of the cars and trucks in the U.S. hit a record high of 10.8 years in 2011. But in the last year, vehicle sales have accelerated, apparently fueled by slight improvements in the job