Originally Posted by
High Wing Drifter
So if Avgas is 44% carbon (8/18)...
Therein is your error. Gasoline (and I presume similarly, AVGAS) is about 87% Carbon, by mass Most of the rest of the atoms are Hydrogen, which don't weigh nearly as much, despite being more numerous!
0.72kg at 87% = 0.63kg of C, which, multiplying by 44/12, converts to 2.3kg of CO2