A casual Google suggests that 20mph is possible for sprinting over short distances. So maybe.
I’d be really surprised if your treadmill goes fast enough. Most standard home treadmills have a max speed of 10 mph, and higher-end gym models top out at 14 or 15 mph.
Of course if he tops off before that we have an answer … 
FWIW I would.
Yes, this is probably what is happening.
Running 15 mph for 2 seconds, you’ll cover 44 feet. If the GPS is off a bit, and off in different directions at each sample, then it may think you went 60 feet in 2 seconds, which is 20 mph.
Times when it is off slow don’t get noticed, because the watch highlights max speed. The average speed is probably reasonably accurate, because over time (random) errors will cancel out. The max value specifically is showing you the top outlier speed, so it is going to reveal the result with the largest too-fast error.
Say the fastest people can run 100 metres in 10 seconds. 20MPH is like doing a hundred metres in 11.2 seconds. So it is possible for some people over short distances. I’ve probably done it or come close, maybe not in the last three decades.
Nevertheless, you are always travelling much faster than that. Maybe your watch realizes:
Consider the movement of the earth’s surface with respect to the planet’s center. The earth rotates once every 23 hours, 56 minutes and 4.09053 seconds, called the sidereal period, and its circumference is roughly 40,075 kilometers. Thus, the surface of the earth at the equator moves at a speed of 460 meters per second–or roughly 1,000 miles per hour.
As schoolchildren, we learn that the earth is moving about our sun in a very nearly circular orbit. It covers this route at a speed of nearly 30 kilometers per second, or 67,000 miles per hour. In addition, our solar system–Earth and all–whirls around the center of our galaxy at some 220 kilometers per second, or 490,000 miles per hour. As we consider increasingly large size scales, the speeds involved become absolutely huge!
The galaxies in our neighborhood are also rushing at a speed of nearly 1,000 kilometers per second towards a structure called the Great Attractor, a region of space roughly 150 million light-years (one light year is about six trillion miles) away from us. This Great Attractor, having a mass 100 quadrillion times greater than our sun and span of 500 million light-years, is made of both the visible matter that we can see along with the so-called dark matter that we cannot see.
Now I have an ear worm.
Just remember that you’re standing on a planet that’s evolving, and revolving at 900 miles an hour…
And how amazingly unlikely is your birth…
A good buddy of mine gold medalled in the Ontario 100m championships in high school. I was slower, of course, but he once timed me at 12.7 seconds. It’s not impossible for the first part of that I was doing twenty weird American units…
C’mon, guys. If RS is actually going to take us up on our suggestions, we have to be MUCH more creative! I understand that the most reputable scientists advocate squawking and flapping your arms like a chicken during any testing is essential to obtain the most reliable data!
Only if you have the proper music.
And it can be seen here.
More fun facts! Top speeds of the world’s fastest footballers, similar in activity I’d guess to Ultimate.
Fastest of the fast once is recorded at what comes to almost 23 mph.
I belatedly realized that I neglected to link my above quote. Sorry. Pray there‘s intelligent life somewhere out in space. 
Any follow up?
Thanks!
I threw my back out! I couldn’t play at all last week. This week was better, but I’m still super stiff.
I did turn on all of my GPS sensors this week. According to the watch, my max was 17.5 mph. I didn’t do any real sprints though. Hopefully, I’ll be 100% next week and I’ll try again with all the sensors on.
Heal well!
Thanks!
Ok, today I was running pretty well, but we were playing on squishy grass instead of the turf field. My watch said 19.2 mph for my peak (my heart rate reached 173 bpm at some point, which is pretty high for me). There’s a pretty cool graph where you my speed peak out, and shortly after, my heart rate jumps. After my supposed 19.2 mph, it got up to 163.
We’re playing a New Year’s game tomorrow, probably on the turn, but I’m sure I’ll barely be able to move after playing today.
So maybe you are that fast …
I mentioned that I played on New Year’s Day, too, but my back was killing me yesterday. I played anyway, and had a couple of short, painful cuts, chasing down the disc or trying to catch up on D. Anyway, I was running much worse than the day before, and my max speed was 15.1 mph.
So, the watch is at least directionally correct – lower top speeds when I can’t go as fast, higher when I’m running well, even if it’s not necessarily absolutely accurate.
I think we’ve gone away from any FQ here, so I’ll stop with the weekly updates. I don’t know if my back will recover by next Sunday anyway – I’m really in pain today.