I have a $10 schwinn from target.....dead nuts on as long as you accurately measure your tire. My only beef compared to a stock speedo is that it isn't "real time", mine only calculates every second or so.....so you can jump from 35 to 45 and have a hard time reading in between if you you are mashing. If you were to buy a really good one I'm sure it would be more "real time".
$10 worth of awesomeness if you need a cheap accurate way to check your speed. I actually was getting ready to get another one here real soon.