I find the after market dippers for the 152/172 to be pessimistic (which is good). I flew a 152 back from somewhere yesterday and dipping the tanks before take off gave me 4 galls per side or 30 litres. I put 40 in before take off and then another 29 to full when I got back. I probably burned around 20 litres on the trip so taking into account unusable fuel it was around 13 litres out on usable fuel when I initially dipped it.