Suggest that for a better comparison, you should fill the tank, zero the Number 2 mpg reading on the dash, and drive normally. The number 2 reading gives a running average, so that by the end of the tank, you'll see what you've averaged throughout. It's a bit more objective than the running average.
If you have an iphone, I recommend you invest in 'Road Trip' from the app store. You can get a lite version free. Input each tank as you go & it will give a running average. It also keeps track of costs, fuel, expenses, servicing, tyres etc. A cracking app. I've been running it since March 2010 and my running average is 30.2mpg, best of 34.6 from a tank, and a worst of 27.57mpg. I have seen 40.1 from the instantaneous reading.