ORIGINAL: Dorsal

60 mph is 88 feet per second. ([60 miles per hour X 5280 feet per mile] / [60 minutes per hour X 60 seconds per minute] = 5280/60 = 88 fps)
Simply mark off a 264 foot-long pair of markers on your runway and fly the airplane past them. If it takes more than 3.0 seconds to fly the distance, the plane is flying slower than 60 mph.