What plot? 65mph was the max speed limit in the US until 1995; and speed is affected by several factors, which includes the weight of the vehicle (& cargo) relative to the engine power output, coefficient of friction of the roadway and tires, wind speed & direction, and of course mandated safety standards and or engine governors. Has anyone ever gone on Test Track alone? With no one else?
My guess in this case is that you can strip off most of that technical speak and just say that the system may be throttled to about that speed, for whatever reason perhaps the car's drive system slightly misses 65. I'll wager that the vehicles should hit about the same speed whether they're loaded or empty, the speed is probably tightly controlled. The electric motors are probably capable of going faster, although it would probably heat them up more. I really thought I got a good GPS track of the outside portion of that ride, it should have allowed me to get a fairly accurate speed for the second or two before you pass that speed sign. But.. I've looked and looked, and all I can find is a partial track that doesn't get above 50 and seems to be the portion when you're heading away from the main building, not the speed run coming back. Perhaps they intentionally set the speed to be slightly under 65 to add to the suspense, to give you the feeling that they're pushing the car to try to get the last bit of speed out of it, the feeling of having that little bit more to go just before you hit the banked turn.
I love Test Track. Just wish it was longer. My son loved the environmental tests .... when he went through the heat lamps he'd start screaming (joking around) "HELP! I'm burning !"
the plot is simply this: how do we know we are actually going the speed they are telling us and the speed ISN'T just a random number programmed into the readout? if it is a true radar system, it seems to pickup a small vehicle a fairly long distance away. i suppose it's possible because it's a straight shot with nothing else moving, and yes.... this was all tongue-in-cheek.