Keith Marshall earns record mark in Outsiders' running back metric

Could the 2016 running back class be as historically great as the 2014 wide receiver class? That's not the conventional wisdom, but this year's running backs are historic in at least one metric that helps predict NFL success: speed score.

This year's top prospects -- Ezekiel Elliott and Derrick Henry -- both put up historically great speed scores. But they weren't as historically great as Georgia's Keith Marshall, whose 40 time of 4.31 at 219 pounds set a new all-time record (our information goes back to 1999) with a speed score of 126.9. And this year's speed scores were not just significant because of those three performances. They were also significant because nearly every running back at this year's combine put up a speed score that is historically better than average.

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 both a back's official time in the 40-yard dash and his weight to produce a size-adjusted measure of his speed using the formula (Weight * 200)/(40 time^4).