Here is my design for a horse speed tester, a schematic has been added. If you want to see how to build it yourself , copy it from the video.
It can be very helpful on a server, or just singleplayer, to test your horses. Its easy to build and quite small.
Speed 1 - 7, donkeys usually get 1 :P It can help you tell, will you win against your friend in a race, or how good is your horse, and is it better than others. There are some really fast ones too!
Diamond if you like it!