...but we know mathematically that a 100mah car should run twice as long as a 50mah car....
True, IF we're talking about batteries that were charged to their prospective capacities...Unfortunately (I'm sure you'll correct me if I'm wrong here
) I believe some of the rated capacities are based on C/10 or even C/16 charge rates. Even if you found someone that had equipment that could put out such miniscule juice, I have to wonder if anyone would SERIOUSLY consider waiting that kind of time to play with their little cars.
I think perhaps the 'best' test is to see what the highest Cx can be used to still achieve the rated capacity.