Appraiser Donald Osborne, who moonlights as an opera singer, sat down with Jay Leno to settle a debate between three cars that each inspired a hit song, including the 1952 Lincoln Capri immortalized in “Hot Rod Lincoln.” Only one of those three cars turned its musical fame into real collector-market gains. Nostalgia and rarity don’t always move together, and this roundup only proves it further with dispatches from a London concours and a Chicago muscle car show. Here’s what actually separates a beloved song car from a valuable one.