Well.... my reply is trying to point out to you that there is no consistent relationship. One will lead the other for a short time and then the relationship will switch. The short time can be seconds or minutes. It depends on the time of day, the size of the spread between them and, as DaveN pointed out, liquidity. Using the cash along with tape reading can yield profits, but one cannot say that the cash leads the futures or vice versa. It changes and it over-corrects (which yields an edge for a very short period of time). Your ability to catch this inefficiency depends on your speed, software, your connectivity and risk tolerance. You are competing with guys who are sitting on lines direct to the exchange and many groups that have machines to exploit the inefficiency.