Mickey Thompson hired MacDonald to drive one of his radical low profile rear-engine race cars in the 1964 Indy 500. Thompson's Ford-powered racers were specifically designed to run on 12" tires and made their debut in the 1963 Indy 500 under Chevrolet power. The cars became known as "roller skates". They were far ahead of their time, but badly designed and difficult to drive. Graham Hill tested the car before the '63 Indy race and refused to drive it because of its poor handling, a condition made worse for 1964 when Thompson was forced to completely redesign the cars to accommodate the new USAC-mandated minimum tire height.