Sleepers, red flags among draft RBs

Damien Williams topped this year's running back class with a Speed Score of 113.2. Matthew Emmons/USA TODAY Sports

Is it possible that scouts have this year's list of running back prospects completely upside down? That's the implication of Football Outsiders' Speed Scores, which rate a number of likely undrafted free agents as strong NFL prospects while suggesting that one of this year's most highly regarded prospects could be among the biggest running back busts ever.

Introduced on ESPN Insider back in 2008, Speed Score is Football Outsiders' metric for evaluating running back prospects. It's built on the simple idea that, because smaller backs tend to run faster than larger backs, we should be more impressed by a 4.5-second 40-yard dash from a 220-pound back than the same clock reading from a 170-pound back. As such, Speed Score incorporates a back's official time in the 40-yard dash with his weight to produce a measure of his speed given his size, using this formula: (Weight x 200)/(40 time^4).

The average running back who makes it to the NFL will have a Speed Score of about 100, with most running back prospects falling between 85 and 110. (Since there are more running backs in the prospect pool each year than there are spaces in the NFL, there are more running backs each year under 100 than over 100.)

Obviously, Speed Score is not a perfect measurement. It doesn't account for a back's agility, or his ability to read holes, or his receiving ability. Nonetheless, it's been a good guide as to which running backs are more or less likely to have success in the NFL based on their draft positions -- and it gives us an idea of potential sleeper and red-flag prospects in this class.

Which prospects had the best Speed Scores this year?