I know this thread is only, but nobody answered the question about ground speed...
One way is as follows (I'm open to other ideas):
1. Mark off the distance traveled in 20 seconds.
2. Count the number of yards (3 ft steps). If you are walking behind a mower, just count the number of 3 ft steps you take in 20 seconds. Be sure your step is 36". If you are on a rider, then mark the "start" and "end" points for 20 seconds of travel, then determine the distance and get the number of yards (3 ft steps).
3. The mph is the number of yards (3 ft steps) divided by 10.
By example, if you are using a hand mower and count the number of 3 ft steps you take in 20 seconds as 35. The speed is 3.5 mph.
This is very simple. Yes, I know for the purist, the units don't work. I think the actual time to make the numbers exact is something like 20.23 seconds. I'll spare everybody the torture of the math to show how it works, but will share it if somebody asks. I worked it out a couple of times in past years, but suppose my brain will still work well enough to do it again.
BTW, this was published in a John Deere farmer's handbook. I remember seeing it in the early 50s. Does JD still publish an annual booklet to give to their farmer customers? Maybe the formula is still given in that handbook.