Hit the 'TRIP' button to scroll through the five parameters (Trip--MPG--Ave Speed--Journey (time)--Range.
You'll find the Range function gives a more sensitive reading of your economy.
The MPG readout is an average since it was last reset, so once you have a good few miles on it, it changes very little, even though your actual MPG may be changing significantly.
The Range function tells you how many miles you will get from the remaining fuel at the current MPG figure, so it'll show up MPG variation much better.