Don't just use the avg fuel economy reading on the site. The best way to monitor your car is to see the full graph of fuel economy over a long period of time.
If you notice some tendency for higher fuel consumption in the mid or long run, you can take decisions and or have your car checked.
If you are trying things to get better fuel mileage, make the change and try it out consistently for varios fuelups (at least three or four) and then check against the past.
Don't just use one fuelup as a measure as even if you are careful to fill up to the same level, it's still not precise enough to guarantee you a good reading.
by schmiedel on May 18 2011
this tip works for 97% of voting Fuelly members.