The Nissan Path

I am new to the PF world but I have noticed that my 2008 R51 2.5l Diesel is only reading about 17mpg average on the Nav Screen. I haven't done any long journeys yet so don't know if this will change when I do but I was expecting a bit more mpg than this from a a diesel. Any diesel owners, is this normal? I was expecting it to be thirsty.....but not that thirsty. Also I see a lot of members on here talking about getting a mpg reading off the dash where you adjust the brightness of the dash lights. My screen there only seems to show me a trip. Is the average mpg only showed on the Nav screen?
Thanks