Thank you..just the info i needed

What is the constant number for or mean?

What about purchasing a digital speedometer that runs off of GPS and works off your cigarette lighter. Gives a accurate ball park speed and costs around 30 bucks.

i wish we had litre version for europe farmers

You can do this by just filling your tank and spraying a known sized area at a constant speed and noting how much water it took to cover that area. lets say you drive 8 miles per hour and it takes 5 gal of water to cover 0.5 acres. That means it takes 10 gal of water to cover 1 acre at 8mph. Add the appropriate amount of water for the number of acres you need to cover and the appropriate amount of chemicals for that water amount.