Not taking sides in any way, but 4.81 seconds in the 40 yard dash is an AVERAGE of 17 miles per hour.
In 2008, Usain Bolt ran the 100 meters in 9.69 seconds. During the 1st 10 meters of that race, he covered that distance in 1.85 seconds which calculates out to 12.09 miles per hour.
At the 20 meter mark, he was running at 21.93 mph as it took him 1.02 seconds to cover the second segment of 10 meters. He didn't get up to full speed until he was beyond 30 meters.
By the time he reached the finish line he was cruising at just under 27 miles per hour.