I am soon to recieve a John Deere 71 used Planter. I do not have a JD manual yet, but found a Yetter manual on line. I have read and been told that Yetters and John Deeres are alike. When I got to page 17 of the Yetter manual it lists MPH as part of the chart/equation for plant population. I can't see what ground speed has to do with plant population. What am I missing??? I am sure the Yetter manual is like JD's as the paint on Page 17 is John Deere green instead of the customary Yetter black. I don't plan on planting at such a high rate of speed that the press wheel leaves the ground or i become airborne..