Roberts continued his road racing successes in 1975, winning three out of four races in the 1975 Transatlantic Match races. After having won the national championship in 1974, Roberts faced an increasingly difficult battle in dirt track races, as Harley-Davidson continued to improve their XR-750 dirt tracker while Yamaha struggled to maintain the pace. Roberts made up for his bike's lack of power with an almost fearless, determined riding style. He battled Harley-Davidson factory rider Gary Scott throughout the 1975 season but mechanical breakdowns hampered his title defense. He had been leading the Daytona 200 when mechanical problems yielded the victory to his Yamaha teammate Gene Romero. At the Ascot TT, Roberts battled from 17th place to take the lead before a broken sprocket ended his race. Roberts' fearless riding style was highlighted at the Indy Mile Grand National. In a desperate effort to keep Scott within reach in the points chase, Yamaha wedged a Yamaha TZ750 two-stroke road racing engine inside a dirt track frame. On a bike that was considered unrideable due to its excessive horsepower, Roberts came from behind on the two-stroke, and overtook the factory Harley-Davidson duo of Corky Keener and Jay Springsteen on the last lap for one of the most famous wins in American dirt track racing history. Afterward, Roberts was famously quoted as saying, "They don't pay me enough to ride that thing". Despite accomplishing another Grand Slam, this time in only one season, Roberts lost his crown, finishing second to Gary Scott in the 1975 national championship.