Outstanding response by balsaeater.

The short answer to your question is there is little or no corollary between engine size and nitro content. There are many boat guys running OPS and CMB .90's on 65% nitro and an OS LA 10 will run fine on 10%. Top fuel cars are 500+ci and run 85% nitro. You cannot look at an engine and say "It's a .40 and needs 15%". You cannot assume because an engine is a _____ it needs a certain amount of nitro.

With the above said, there is complete connection between engine size, fuel consumption and the cost of fuel considering nitro content. I'm certain the makers of the Moki 2.10 and the Super Tigre 3000 had fuel burn rates and the cost to feed these engines in mind when they designed them to run on low or no nitro fuels. In general, all glow engines would run better with 10 to 15% nitro if they were designed that way however, in the case of larger engines, they are often designed to run on lower nitro fuels to keep operating costs down. Since fuel consumption rates for smaller engines is much lower, they are often designed to run on higher nitro fuel.

A few things to remember...

Even in todays inflated world, Methanol is still only $3.00 a gallon.
Nitro is 20 - $40.00 a gallon.
The cost to feed a Moki 2.10 on no nitro is less than the cost to feed a YS .91 in a heli running 30%.
A gallon of 40% fuel in a Cox 1/2A will last about 250 flights. Flying 4 flights a weekend equates to about 1 1/2 years of flying on a gallon of fuel.
A .90 heli holds about 20oz and a gallon of fuel last about 5 flights.
Nitro is stupid expencive in other parts of the world and engines designed for use outside the US are almost always low nitro... Large or small. Think Webra, Rossi....

Bill