Fuel calculations

Hi all ,I just got back from a short trip Penrith to Armidale and back
and as we were driving I asked a mate who was also driving with a car load of people sleeping how do we work out Ltrs per 100 kms, well didn't I open up a can of worms and at the end I just said forget it I will ask my mates on E/O and this is what I'm doing now,
I have this info but I'm not sure how to use it and no I don't normally drive my 4B to dead empty I was just curios.
I drove 920 km to empty both tanks
I put in 132.46 ltrs of diesel
I paid the nice man behind the counter $157.50
I think that if I was to say 132.46 Div by 9.2 = 14.39 per 100 KMs
would that be correct?
I'm asking this because we are going on a larger trip and I have been told the average trip around the block with no side trips is about 25000kms so I thought that's about $167.00 per 1000kms
25000 x 167.00= $4175.00 ??
I know I must have stuffed something up because it sounds too CHEAP!! LOL!

Regards John.