the best source we've seen for ratings in recent times is a D. Harmon article. He laid it out somewhat vaguely but with enough info to show that BYU does not own the market. The ratings separation was relatively small in favor of BYU. They lead on a point basis but it is not a dominant lead.

The other somewhat reliable data comes from recent bowl games that Utah and BYU have been in with similar TV distribution. Even when dropping the Sugar Bowl Utah's ratings have come in higher than BYU. A lot of factors can go into that including opponent but it shows that there is some great discrepancy between though two.