Ok, First off sixpax2k9 is right. The gray wire is connected into some sort of pot that is adjustable under the black shrink wrap. You can move this pot and notice a difference in how fast the motor is spinning. My was about three quaters of the way up. I turned it all the way up and could notice a difference in how loud the motor was and it seemed to be spinning faster. I did this several times while the motor was spinning and could hear and see the difference. Here is the weird part of the story. I connected the gray wires together which is connected to the throttle by way of the pot under the the shrink wrap as sixpax2k9 said. When I did this my throttle would not work at all. I disconnected the gray wires and then moved the pot back and forth and that is when I noticed the differnece in the motor speed. So here is my take on this. I have an aftermarket controller that maybe doesn't work when the gray wires are connected together,but it seems to me that the pot is already working to adjust the motor speed with or without the gray wires. This circuit makes alot of sense from x-treme's stand point. They can just adjust the pot to make sure the bike doesn't travel over 20 mph. With all the variables in electronics you would need a circuit like this to adjust bikes that are a little to quick when produced at the factory.
I then took the bike out for a spin I registered 29mph as I have before but this time it was on a less than fully charged pack. Before I did this I took the bike to store on the same not fully charged pack and it was recording a speed of 27mph.
If in fact this does adjust the speed up a couple mph provided it isn't already turned all the way up that would be great. My problem now is I wanted to use this to slow the speed of the bike down for certain situations. Doesn't seem like that will be the case since I personally can't even connect the two gray wires together.
I will repost a speed test on fully charged pack later this weekend. If the 2mph is a real increase I should see maybe 31mph top speed.