Yes, it’s the preseason. And, yes, it’s a small sample size. But midway through the exhibition season, the Dolphins’ best running back has two regular-season carries in the past three years and is on his third NFL roster in 11 months. His name is Isaiah Pead, and he’s doing whatever he can to force Adam Gase into a very tough roster decision. For the second week in a row, Pead was strong when the lights came on. He led all Dolphins in yards from scrimmage (78) and had two of the team’s three most explosive plays in Friday night’s 41-14 preseason loss to the Cowboys.