While there's no doubt the Cash for Clunkers program drove buyers to dealers' lots this summer, a new report questions which country's cars they most often drove away in.

According to the report from the University of Michigan's Transportation Research Institute, nearly 85 percent of the trade-ins for the program were from General Motors, Ford and Chrysler. However, those companies sold only 39 percent of the new cars moved by the Clunkers program.

About 8 percent of the trade-ins came from Toyota, Honda and Nissan, Japanese manufacturers that accounted for 41 percent of the new car purchases.

The institute also suggested that buyer loyalty for Detroit's Big Three took a hit during the summer program that consumed $3 billion in U.S. taxpayer funds.

About 68 percent of owners who traded in Toyota, Honda or Nissan vehicles replaced them with another from those Japanese brands. Loyalty was highest for Toyota, which saw repeat buyers 44 percent of the time, as compared to 30 percent for Honda and 19 percent for Nissan.

But when it came to the Detroit automakers, only 43 percent of consumers who traded in a vehicle from one of the Big Three manufacturers bought another from those same brands. General Motors led in brand loyalty among the U.S. automakers with 32 percent repeat buyers, with Ford at 24 percent and Chrysler at 11 percent.

As for those who switched, the Japanese manufacturers got 38 percent of buyers who traded in a Big Three vehicle. Toyota did best with 18 percent. General Motors, Ford and Chrysler got only 12 percent of shoppers who traded in a vehicle from one of the three Japanese automakers. General Motors did best with 5.3 percent.