Biffle began competing full-time in NASCAR's top division in 2003, with sponsorship from W. W. Grainger, who had previously sponsored him in the Busch and Truck Series. He earned his first win in the Pepsi 400 at Daytona that season and finished second to Jamie McMurray (who would later join him as a teammate at Roush) for Raybestos Rookie of the Year. Biffle placed 20th in the final points standings.