Actually considering selling my 2009 xc70(after 3 weeks ownership)...Never thought that the mpg's would be this poor... Getting 18 combo.
Now I have to figure out how to sell a car that I bought from the dealer (CPO)...

 

Has the car been tuned up? New plugs? Depending on your area, average speed & length of your trips, that may be decent.

 

What is the engine in the 2009 XC70? Is it the approx 240 hp 3.2L inline 6-cyl?

My wife's FWD XC90 has this engine and I find it is not especially fuel efficient, maybe 14 to 16 mpg around town and 22 to 24 mpg on the road, and this is with it driven gently.

The same engine in an XC70 will do better than the XC90, but AWD also takes a toll.

I understand that you are shocked and disappointed, but my advice to you is that unless the dealer will take this vehicle back and refund nearly all your payment, you should keep the car at least for 6 months or a year while you consider what you could get that would meet your performance reqiurements and fuel efficiency goals. Don't react hastily--it could cost you a lot in transaction costs.