I have a john deere model 318 tractor with a 38 inch mower deck. I just bought a new belt part#gx20072 exactly the same as the one that broke. I can not get the new one on . Its like 4 or 5 inches short. What am I doing wrong?

I would look up the service manual for your mower. Chances are that the belt is meant to be under tension so you will have to move a pulley somewhere put on the belt and then retension the belt by moving the pullet back to its proper place. Just a guess.

For either the drive belt of the deck belt, there will be an idler pulley. The drive pulley can be pulled back by pushing in the clutch pedal. The pulley for the deck can be pulled back by turning the blade lever to off.