I've been reading this site for a while and have found it very helpful, very sweet site.  Now, I have installed the trailtech vapor on my '07 gs.  I did all the measurements for the wheel diameter to put into the unit.  My question is doesn't it have an effect on how close to the center of the wheel the magnet is located.  For me, I found the best position to be fairly close to center, i.e. shorter faster rotations as compared to moving away from center, longer slower rotations.  It seems to me that the speeds are fairly accurate until I reach around 45mph, then it seems like it is reading to slow.  I did a test with a radar gun my brother had, but have no idea how accurate it was.  Any suggestions on some changes that I may need to make would be appreciated.

In your mind, draw a line from the end of the front axle to any point on the rim.  Every point on that line will go past the fork the same number of times during a given time period, or over a given distance.

However, the closer the sensor is positioned to the axle, the longer it will take for the magnet (or whatever) to go past the sensor each time, which may affect the accuracy.  The further out you go, the magnet (or whatever) will be moving at a faster speed and its motion past the sensor will register as a quick "tick" on the sensor rather than a longer "bliiip".

Personally, I would mount the sensor as far from the axle as possible; it seems like it would be easier to "dial in" that way.

While I have no experience with the veypor unit, I have used a bicycle computer that uses a similar pick-up for speed. I had the magnet stuck on the brake rotor. It worked well and was accurate. As a note I left the regular stock speedo hooked up to make the comparison. So to answer your question it is probably pretty accurate. Also the stock gauge is known to be off a certain percentage above 45mph or so I believe. That could be why you think it is reading slow. I'm sure someone will jump in and either agree or correct me if I'm wrong.

Thanks for the help guys.  I just ended up hooking up my old speedometer and running it side by side with the vapor.  Farily easy fix, just had to play with the numbers some.   

those magnetic doo hickeys are cool until a 'friend' super glues a weak magnet to one of your spokes. don't ask how I know.

I used a trailtech on an old 4-wheeler I had... it wasn't the vapor but it worked well.  The only thing I didn't like was the fact that it wasn't backlit at night.  It was pretty easy to do.  You just plugged in the distance from the axle to the magnet OR the height of the wheel & tire.  The wheel and tire method worked a little better I thought, seemed more accurate to me... but that was on a 700R rapter, I had nothing already on the wheeler to compare it with.

post up a pic.. would like to see how it looks mounted on a 07

Ill try and get a pic up tonight and show you where I mounted the magnet and why I had to put it closer to the center of the wheel than I would have liked.  But, it works great.  I am still in the process of having a custom mount made for the actual computer, right now it is just mounted on my handlebar.