That’s what I paid for a Zipp 400 front wheel last night. Came with a pretty new Conti sew up, too.
Today I took it out for a test ride to see how it handled (32 spoke box rim in the back). Just noodling around on my street, my Polar read 40kph. Riding “easy” in Z1 it read 53kph. Yes, it was displaying twice my actual speed… I haven’t looked at it closely yet, but I’m gonna have to check for a 2nd magnet when I get home. 
did you forget to readjust the wheel diameter on your polar?
I didn’t bother for today’s ride, but that certainly wouldn’t explain a reading of twice my actual speed. I’ll measure and calibrate it for tomorrow’s ride.
You paid a hundred to double your speed? I could get a second magnet much cheaper than that!
The magnet I stuck on it came from my spare parts bin, so it was “free” (I must have paid for it at one point in time). There’s only one magnet on the wheel, so I’m going to measure the wheel and re-calibrate the unit before I head out in a few. Will post the results when I get back.
I do rather like the deep dish wheel though! 
I got that kind of a kick when I got my carbon fiber bike pump.
Okay, so the wheel diameter of my “new” Zipp 400 is actually larger than that of my training wheels, so when I recalibrated the watch I had to make it bigger. I’m still reading double speed/milage. I can’t say that it REALLY bothers me, but it is annoying…
I read something about this a while ago. Unfortunately I can’t remember exactly where or what wa steh cause.
I believe it had something to do with teh type of magnet. If the magnet was the wrong size or power or something the pickup would read both oles of the magnet, effectively doubling your speed.
I believe you said this was a “spare” magnet. Have you tried swapping out teh magnaet from the last tire?
A not so well organized thought…
~Matt