[Mr. Pink]

My theory on this is that back in the days with brushed motors and nimh/nicd batteries both power and amount of energy stored to finish 5min of racing was the main concern to make the cars go faster. The solution then was that lighter was faster as it put less strain on the motor and batteries. Now with brushless, lipo and power beyond, the limit now is how to get that power more effective to the ground. And if the tyres will produce more relative frictional force with a higher tyre load then heavier is faster.