ground speed / 4th gear / farmall M

Your tire size, inflation, governer setting, and overall tractor condition will affect your speed. You probably have bigger tires than what the book speed shows.

Then run your sprayer while sitting still with plain water in it and fill a bucket out of one nozzle until you get two or three gallons. Time the fill and measure what you have filled (I use a quart measuring cup) to get an accurate idea of how much fluid you are pumping, times the number of nozzles. For accuracy, I like to run on the relief valve. My sprayer will put out over 60 psi in all conditions so I set the relief at 60 psi and let a little bit bypass all the time so I have consistent pressure and therefore consistent flow through the nozzles.

A little arithmetic and you can easily figure how many gallons per acre you are putting down.