Drop in gas mileage - unknown cause

[TuftsAuto]

Hey all, my 2002 Boxster S has been running at lower efficiency, and seemingly lower power, for a few weeks, and I'm now getting around to chasing down the source of the problem. I noticed that my mileage used to be around 20-23 mpg in the city (27-29 highway) but it has since dropped to around 15 mpg. In addition, there seems to be a slight drop in power as well as a faint unburnt fuel smell coming from the exhaust. There is no check engine light.

Any ideas on how I can chase down this issue? Has anyone had this happen before?

Note that my car did get hit at low speed in the front end a few days ago, but I think that this is independent because the issue popped up weeks ago.

Thanks for your help!