To do a reliable check you need to brim the tank, then the next time you fill it you will be able to work out the consumption more accurately. There are many factors that affect the economy, they way you drive, the sort of journeys you do, the traffic speed and the temperature, are all thes things the same as they were before?

Does your vehicle have the euro 4 fitted fluid charged particulate filter on it? If your not sure have a look underneath and see if the l;arge under car exhaust box has 2 small pipes running to it, similar size to brake lines. If it does then it has the filter Im talking about. This system causes poor MPG when it needs recharging. To recharge it you simply drive 30 miles at 50 mph in the highest gear possible. That give the vehicle a constant to work off so it can recharge the system. A friends car was the same, one garage quoted over a garnd to fit a new filter!! Main dealer told him to do the constant drive thing.