Speed Scores for 2012.
Football Outsiders published its speed score metric for 2012. It hasn't been as accurate as before but its still a nice metric for mid-round backs.
The formula is Speed Score: (weight * 200)/40 time^4.
Top 5 Speed scores
Lamar Miller 113.1
Robert Turbin 108.3
Bernard Pierce 107.3
Doug Martin 104.1
Cyrus Gray 103.2
The rest of it is on ESPN Insider, but you could calculate it yourself though.
Average speed score is around 100 and average score for a first round back is 112. Only 7 backs managed a score above 100.