The GT ProSpec class used to have no weight rule and no motor timing specs so most fast guys had their cars down to low 1200's in grams and the Reedy Sonic motors cranked up to 56-58* of timing for a KV rating of around 2650-2750. Cars were fast and fun to drive but the lack of rules just pushed guys to press harder. One day you're faster and next week they come back with a higher timed motor and now they're faster. In a class with such a high spec FDR its all a motor game.

With the new timing rules in effect (46.0* per HobbyWing) the 21.5 motors are turning a much lower KV (maybe 2200-2300), add the minimum 1400g weight rule and these cars are now sluggish and slow. By going to the 17.5 you get the KV back up but then limiting the RPMs via the ESC gives Tamiya the control to set the pace of these cars based on the event/track size. ETS series in Europe has shown great success now running the HobbyWing platforms (Justock Motor/ESCs in lower classes, and higher end 100a ESC and G4 motors in the expert stock classes) and setting different RPM caps based on chassis (Fwd, TC, F1). With the RPM limit, its very easy to tech and completely levels the playing field top speed wise.