Play the Stock Car level. When driving past a certain speed, you will hear a sound that will confirm that the Super Mustang has been unlocked.
Go to the Ford Collection and select "Time Attack". Choose the Temple Ruins track. Use the '56 Ford F-100 as your car. Once the race has started, turn around and go as fast as possible, then go up to the big hill just after the starting line (still driving the wrong way). You will fly at heights of over 50 feet. Try different cars for even bigger heights.