Nitromethane, commonly referred to as NITRO for short, is derived from propane. In itself, nitro is not even extremely flammable as one might think. The real power producing potential of nitromethane is that it carries additional oxygen with it into the combustion process. It acts as a fuel too, but the molecule carries oxygen along with it. In a way, nitromethane is sort of like a chemical "supercharger" for an engine. It helps get more oxygen into the engine to help burn more fuel and produce more power, but since the flame speed of Nitro is much slower than that of Methanol, adding Nitro the mixture slows the flame speed. This should not be a problem until you hit very high rpm’s. There's a moment where the engine cannot pickup more rpm’s because it needs to burn the mixture at much higher speed in the combustion chamber. 30% Nitro fuel also burns at a lower temp so it will give you advanced timing and therefore more low end power.
With lesser nitro, you may need to lean it a little more and your fuel mileage will be a little better, temps will vary and combustion chamber and carb setup need to be revised, but... more nitro doesn't necessarily mean more power, but yes more fuel consumption and less runtime for the same CC’s.
Higher nitro content does not cool the engine, in fact it creates more heat. Basically, more nitro = more oxidizer to the methanol which in turn creates higher combustion pressures which create more heat.
So more low end power, less runtime = higher Nitro contents
More high speed power, more runtime = less Nitro contents

AFM