If the car's running really rich, you'll see a puff of fuel out the exhaust when you accellerate hard. Easily seen if you get someone to drive behind you. That can affect fuel efficiency.

There may be some sort of leak. But the fuel wont just disappear, you'll be able to see it on the ground or on the outside of the engine... has a distinctive reddish colour to it. Dont want it to drop down near the exhaust or it might get a bit toasty. Would have to be a serious leak to affect fuel efficiency though.

Just a couple of ideas.