I'd stronly recommend the Etrex or Geko from Garmin. They both record max speed along with many other features. Accuracy is .1mph. They also have a refresh rate of once per second.
This is what I have
http://www.target.com/gp/detail.html...sin=B00007MMNS
I paid $79.99 CANADIAN for it. The store I bought it from reduced the price because they came out with the Geko 301.
This is the cheaper larger brother.
http://www.target.com/gp/detail.html...sin=B00003WGP5
Ryan