Did you notice worse gas mileage while it was going out?
I've noticed my gas mileage getting poorer and poorer over the last 2-3 months despite my car running great, and I've just found out my clutch is getting real bad real fast and needs to be replaced.

Do you think this has anything to do with it? It makes sense to me, the clutch slips takes more revving to get the car going = bad mileage.

Then again, this is my first manual, so what do I know

 

I would agree with you, just like you said, you need more revs to get the car going, therefore using more gas.