Try this method for a reasonable approximation of ground speed in miles per hour:
1. Drive a post into the ground.
2. Write down the RPM you will drive at.
3. Get a running start at that stable RPM; drive in straight line past post.
4. Click stopwatch (phone, wristwatch) when passing post.
5. Time 1-min.
6. Stop at end of 1-min (or observe a marker or drop a glove to the side).
7. Measure from original post to 1-min end marker.
8. Multiply number of feet times 0.011.
9. Repeat and recalculate; go each direction twice.
10. Average the answers and you have your ground speed in MPH.
The more times you do this the more accurate the final answer.
You can twiddle the numbers like this:
(Any number feet)divided by(how many minutes)times(0.011) = MPH
The answer becomes more accurate the longer distance you drive and the precision with which you click stop-start.
It helps to have a long measuring tape; the longer the tape the better the answer.
If you have only short measuring tape, use 15- (0.25-min) or 30-secs (0.5-min). [[Got to use "minutes" not "seconds" to make this process work.]] But the final answer will be less accurate.
This basic approach is one usually found in publications from state universties ag departments when discussing calibration of sprayers.
I use this every season when recalibrating sprayers. I even have a marked course by tying flagging on a fence. Fast, simple, and accurate enough.
Please post back your experiences so we may all learn.
