Hi,
The KEY to physics simulation may very well be not trying to set it up with realistic numbers... Some might be able to achieve this, but this is in my opinion too much trouble to get into that path. it's a general rule I feel for any real time application, not just the physics. We cheat with modeling and texturing to avoid too much details in the mesh, we cheat with internal structures to achieve the effects we want like parenting the moon to the earth and not having forces attracting the moon as it orbits around the earth... in the same spirit, let go on real physics numbers and experiment.
you need to achieve a simulation that feels real, but that it is not necessarily using real numbers.
So, your rc car will have the weight it needs so that it performs like you want, it might means a mass of 1... or 10, but certainly not the exact weight of the car itself. Same with every physics set up really.
I know few guys that can get it right with the real numbers... but man... you don't want to go down that road...
real size cars vs rc cars is not different at all really, it may seems so because all frameworks actually tend to be for real sized car. Any car or vehicle size is really about tweaking values.
So don't give up! When I created my excavator as a typical example, none of the values ar correct, my rig doesn't weight 4 tonnes... all arms weight 0 to begin with and do not react to gravity. only the under frame weight like 20... and the bucket around 2 if I recall properly... And the numbers are use for springs, damping, actuators forces are experiments, not real numbers. And to tell you the truth, this was the part I spent the most time, tweaking the physics so that it behaved like I wanted. The rest is nothing really...
http://www.fabrejean.net/projects/excavator/bye,
Jean