So what we did over here is we said: ok, for a 6-HP tractor, I used their number, we are going to have to use 25 hours to do it, it is going to use this much petroleum, assuming that is uses 23.5 litres of gas or petroleum over 24 hours, and I just put 20% for that number for saying how much energy had to be used to maintain that vehicle over that amount of time, and if you think about what fraction of this vehicle's life that 25 hours represents, that fraction times the total amount of energy required to produce that. 