14.7 kg of air is required to burn one kilogram of gasoline, but only 1.7 kg of air for one kilogram of nitromethane.
Nitromethane, however, has a lower energy density: Gasoline provides about 42-44 MJ/kg whereas nitromethane provides only 11.3 MJ/kg. This analysis indicates that nitromethane generates about 2.3 times the power of gasoline when combined with a given amount of oxygen
25% little more power and easier to tune compare to 20%.
20% is cheaper and run longer per tank but not common