The rate of flame propagation does not vary significantly between gasolines with different octane ratings as long as there is no detonation, and the same applies to peak temperature of the flame front, which is where NOx forms. "Premium gas burns slower" is a myth!

To the thread originator - I don't think there is a "problem" with your converters, but it will help in diagnosis if you would post the entire test results for both 15 and 25 MPH including the 02 content.

Duke