It makes no difference GAS MILEAGE wise.....power wise a SMALL difference on engines that NEED higher octane. Its just throwing money away to use it really on any engine. The slight difference in power is not worth expensive gas nowadays. Its not possible that the gas mileage would be affected. The knock sensors will adjust to the lower octane and just reduce PEAK power somewhat noticably. It CANT affect gas mileage by more than the TINIEST amount.
Then my suggestion to you is to conduct your own experiment and see. As I mentioned earlier, I've done this test on every vehicle I've owned over the past THIRTY years and have always seen the same results: Better mileage with super, lower with regular. And while we're talking about octane, knock, etc., here's and excerpt of something I posted in a different forum that directly relates to fuel and mileage....
Many people report poor or inconsistent mileage with the same vehicle. The big culprit in poor mileage may well be the gasoline blend. Most people think gasoline from Brand X actually came from Brand X's refineries, well, it very often does not. Gasoline is bought and sold on the open market and often not from your expected source. Also, when gasoline is produced at the refinery samples are constantly pulled for testing, much as master beer brewers pull samples, and the gasoline is tweaked in order to compensate for vapor lock, etc. In the end, the gasoline that leaves the refinery is only as good as the chemist who blended it. Finally, people also think that gasoline is produced and shipped immediately, but the reality is very different. Selling winter blend during the summer season (and vice versa) is common, and it's not just Mom & Pop small sellers who buy it cheaply off-season and sell it at those tempting discount prices, the independently-owned national brands do it too. This gas is produced according to demand estimates and stored in massive tank farms, then sold later when needed.
In summary, purchasing gas is more akin to playing the roulette wheel on a rolling cruise ship than anything else. If you can find a station in your area that consistently offers good gas, you're lucky. According to my sources there's two major brands that refine gas very well, and make efforts to ensure the stations that display their name also sell qualitative gas. Why have I not mentioned them?...I don't need a flame war over brands or a law suit from a brand either.
So take a look at where you're buying your gas, and at the very least don't buy it those nationally known discount warehouses or Mom & Pop stores that offer the tempting deals, as the saying "if it's too good to be true..." most often applies. Just food for thought.