if slip is lower than 10 persentile , ballast needs to be removed from the tractor. if slip is higher than 20 persentile , more ballast needs to be added to the tractor. one way to determine slippage is to compare the number of rear wheel revolutions when the tractor is not under load with the number of revolutions under full load. for information on measuring wheel slip, see the article optimize wheel slip to save fuel.