So I'm a Saaber but my mom owns a 2001 V70 2.4t which always has a problem! So anyway I borrowed her car and i started with 1/2 of a tank after 50 miles the tank was down to a quarter. Also seemed to lack power I had my foot all the way down and seemed unresponsive! She had a code p0422(which her mechanic fixed before) then went away.

What could this be? she throws so much money into this car she should of kept her S70.

 

First thing I'd check is the air cleaner. Easy to check, cheap to "fix", and could be the source of low power and high gas consumption. I'm not familiar with the 2.4t, but my T5 (2.3 high pressure turbo) is pretty quick (supposed to turn a 6.6 second 0-60).

The P0422 code is for poor catalytic converter efficiency, so I'd also consider cleaning the MAF sensor (just past the air cleaner box, so you should be in the area anyway). Make sure you use a real MAF cleaner (don't use anything that might leave a residue on the element or you might make things worse).

Both of those a dirt simple, cheap fixes that should be done occasionally anyway, and will help the performance and economy of the car even if they're not "bad". If those don't do it, it'll be a lot more complicated and you'll need to do a lot more diagnostics.