How accurate do you need the speed? I build a TDR to measure and find breaks in cables and I had considered altering it's input method to use laser detectors so I could measure distance via time of flight. The timing chip I use can measure in the 45-90 pico second range so quite accurate and I could measure to about 2cm on average. Laser would maybe be slightly less accurate.
http://forum.arduino.cc/index.php?topic=183770.0