looking at the BFG tracker.
1971 wayne matlock came thru pit 4 mile 470 at 2:22 and shows him passing thru pit 5 mile 622 at an undisclosed time, somewhere between 5:16 and 5:40 by the car before and after him. so just for an easy guestimate lets say he went thru at 5:30
that puts him making it from pit 4 mile 470, all the way to pit 5 mile 622 which is 152 miles, in the neighborhood of 3 hours flat. which means thats around a 50mph average speed over the course of 3 hours. i dont know if thats highway, washes, or what the terrain is from mile 470 to 622 but he was rolling!