It was his experience with single-seater and IndyCar designs which prompted Dan Gurney to hire Southgate for his All American Racers team, based in California, to design some of the second generation of "Gurney-Eagle" USAC racers. The highlight of Southgate's time with AAR was when Bobby Unser won the 1968 Indianapolis 500 race in one of Southgate's Eagle cars. The Southgate-designed Eagle Formula 5000 car also found some success.