Urged on by roaring fans, 43 NASCAR rivals flashed around the flat oval at Virginia's Martinsville Speedway in the Subway 500. Drivers braked at the track's notorious wreck-maker turns, then screamed down the straightaways at speeds of up to 126 mph. Before packed grandstands and a national television audience, each car doubled as a speeding billboard emblazoned with logos like Home Depot, Motorcraft, Interstate Batteries, and . . . Care-Net Pregnancy Centers? Hardly your standard manly racecar ad. But Joe Gibbs Racing Team driver Bobby Labonte indeed circled the track on Oct. 23 in the Interstate Batteries car, its entire...