Hi..generally rc nitro engines have an rpm ranging from 3000-28000rpm (2.6cc-4.6cc) with 1-3.6hp. I want to know how is the max. load considered in calculating the load rpm(ie 3000rpm) of the engine. How does it depend on mass (quantitatively)? How much rpm can it go with vehicle mass of around 2.5-3kg.

In theory the max revs is independant of the mass of the car because top speed is reached when the engine power just equals what's needed to overcome rolling resistance of the tyres and air drag. A heavier car will just take longer to reach the same speed (more mass to accelerate). In practise though, a heavier car will have more rolling resistance and that will slow it down compared to a lighter car. That's all assuming you have the same car and engine to keep everything equal but load it with lead weights or whatever for more mass.