Recently, Football Outsiders published their Speed Scores. Speed Score is an advanced metric that factors in RB's official 40-time from the NFL Combine with their weight.
Speed Score is Football Outsiders' metric for evaluating running back prospects. It's built on the simple idea that smaller backs tend to run faster than larger backs, so 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.
Speed Score measures speed in the context of speed and power. Basically, it's a take on Newton's Second Law, which states "the net force on an object is equal to the mass of the object multiplied by the acceleration of the object.