Why, when my hdd has only 160 gb of files on it does my time machine say it is backing up over 250 gb?
Time Machine only makes an estimate of how much it thinks will backup. That estimate can be wrong by a factor of 2 in many cases when there's nothing wrong at all.
- Why does Time Machine's backup size not equal the size of my hard drive?
- Why is Time Machine over-stating the amount of data that needs to be backed up?
If you get into trouble, you can always exclude the system files (which you will reinstall anyway from OS install, not from Time Machine) and exclude other files if you want to exclude some large folders to speed a backup of the rest of the files.