Can I add a random 5 litres without it affecting my stats?
 

I usually fuel my car from low to full (so, around 90-95% of my 60-litre fuel tank each time).

My dad has offered me a full 5-litre can of fuel.

How can I add this without it affecting my Fuelly statistics?

Just put it in as a partial fill up.

Would it also be the same if you just left the trip meter running, and take away 5 litres from your next fuel up?

Should that not be ADD 5 litres to the next fuel up?

Haha oh yea. I've often considered this scenario before, if I ran out of juice and filled up using a 5 litre jerry can.