DETROIT -- Two big auto dealership chains delivered more bad news for domestic car makers: Many Detroit auto franchises have become practically worthless.

In third-quarter earnings reports Tuesday, Group 1 Automotive Inc. and Sonic Automotive Inc. announced charges of a combined $51 million reflecting declining "franchise value" for stores that sell brands from General Motors Corp., Ford Motor Co., and Chrysler LLC. Franchise value is a measure of the potential profit a company can make from holding the right to sell new vehicles...