This shortcut is right after you come out of the tunnel. Head toward the wall to the left where the gray wall meets brick. Use a blaster mushroom to shoot yourself toward the wall, and right as you hit, jump over the wall. You will be right near the finish line, cutting precious time off your race.
Mario Kart 64 Shortcut Video Walkthrough Series for Nintendo 64
Mario Kart 64 - Luigi Raceway Shortcut Video
