computing (car) mileage

I've been thinking on how best to get an accurate estimate of fuel consumption on my Honda Civic. Here is what I came up with:

• keep a notebook in car
• upon filling up the tank with gas:
• fill the tank all the way up
• note how many litres were purchased; write this down in the notebook, with today's date
• also write down how many kilometres were travelled since last such refueling
• reset the "fuel" distance counter

Figuring out the mileage over the distance travelled since the last refueling thus becomes trivial:

mileage = 100km / distance travelled * litres consumed

This gives you Canadian-style mileage, expressed in litres per 100km.