The first add on is my Scangauge II which I originally bought for my Discovery 3 .It is basically just a display screen the plugs into the OBD port.But it is very handy in what is shows as far as consumption , both real-time and average , range left , distance travelled etc etc . For more info go here : http://www.scangauge.com/products/

As you can see below , I’ve since discovered that you can daisy chain 2 or more displays . This allows you to monitor more readings without having to change the display .