I agree it was odd to get better mileage with the lower octane fuel and have no explanation. I took a look at the kinds of miles - pretty much the usual combination of freeway cruising and neighborhood errands. On long trips, the mileage soars, but that's probably because I'm good about travelling at a consistent speed.
I ain't sayin' what that speed is, but it's not legal and it's fun....
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban