It's that "washer looking thing" that's mounted in front of your front sprocket. Take the left fairing off (if you're sitting on the bike). Then, take your sprocket cover off. To do this you'll have to remove the left side rear set, unless you can slip your shifter linkage off without removing the whole rearset. Once you get the sprocket cover off, you'll see an allen wrech head bolt right in the middle, and you'll see it's got 4 little metal triangle looking things surrounding it, that are attached to a "washer" and that allen wrench bolt is inside those 4 metal protrusions. That's your "speed rotor". The wire going into your sprocket cover is attached to a magnet, and that's your speed sensor. I'm not sure exactly how this works but I'm thinking the magnet picks up the revolutions of those 4 triangle pieces and uses those "pulses" to come up with the speed.
The reason I'm all over this one is because I put a front sprocket on my bike a couple months ago. Read all I could about doing it myself since our Suzuki dealer here is always booked a month out or more and we have no other motorcycle shops around here, let along performance shops. Anyways, long story short, the only info I found about a spacer was to remove it when doing the 520 conversion. I was just swapping in a smaller 525 sprocket, so I left the spacer in. BAD CHOICE. I rode it for a month or so, and everything was fine. Then, on the nicest weekend this year so far, I hear a noise like a soda can flipping up and hitting the header pipes, and look down and no more speedo. Rode it home and checked the Yellow Box connections and used the test modes to find it was getting no input. Then I got nervous. Took the sprocket cover off and there were the 4 little triangle pieces, laying on the garage floor.
So, I had to order a new one. Cost me $10.40 from the dealer and should be here Tuesday. Sucks though, cuz I have no speedo and I can't tell how far I've gone as the odometer/trip meter also use that sensor to rack up the miles.