Read a post a couple weeks ago that someone got 4 miles more per gallon by using hightest. I just tried it on my most recent trip to New Jersey in my 1992 960 Wagon with 150,000 miles. I ran the tank virtually empty on regular(87) on the trip down and got 27.5 mpg. Return trip was on premiun Sunoco(93) and I got 27.8 mpg. My highway speeds ranged between 65 - 85 depending on traffic flow and police coverage. The .3mpg is about 00.1% difference and statistically insignificant in my book. The motor runs smooth. I'll stick with my 87 octane and only give it the high octane when its on sale. Just call me frugal.