Typo / correction to my last post:
It should have said “subtract one gallon from what you read on the C-152 fuel dipstick to determine the USABLE fuel in a C-150 (not unusable).
The logic being applied to this kind of thinking is the POH for the C-152 states 1.5 gal. unusable (0.75 gal. each tank). The POH for the C-150 states 3.5 gal. unusable (1.75 gal. each tank.). The difference in unusable fuel between the C-150 and C-152 (per standard fuel tank) is the C-150 has one gallon more unusable fuel than a C-152 per fuel tank.
If you look at the C-152 usable fuel stick you will note the zero on the fuel stick don’t start at the bottom of the stick. It starts about 0.75 gal. above the bottom of the fuel stick.
Logic would lead you to believe that zero on the C-152 fuel stick when using it to measure usable fuel in a C-150 is really at the one gallon mark.
The problem with this logic is the graduates on the fuel dipstick are not correct and if you are adjusting your fuel load to try and stay within the 1600 lb. max weight (which many C-150 owners need to do) then you will be over weight.
