Is it Plunder Road 1957?
The IMDB states:
At Wells, Nevada, Eddie Harris (Gene Raymond), Frankie Chardo (Steven Ritch), Commando Munson (Wayne Morris), Skeets Jonas (Elisha Cook, Jr.) and Roly Adams (Stafford Repp), rob a government train transporting $10,000,000 in gold to the San Francisco depository
In Los Angeles, Eddie calls his sweetheart, Fran Werner (Jeanne Cooper), and she meets them at a warehouse where they melt the gold and cast it into hub caps and bumpers on a new Cadillac. As they drive onto the freeway, heading for a boat in San Diego, they are struck from behind by a car and the bumpers lock.