If you look on the interwebs you'll find all kinds of hyperbole and controversy surrounding the simple topic of using premium gasoline versus regular. Inevitably someone will bring up the notion that "if you can't afford an extra $5 per tank, you shouldn't be driving the car!" as if throwing money away is somehow irrelevant. I've always been a little skeptical about claims of premium fuel superiority. My 2006 NC Miata with a naturally aspirated, and honestly pretty craptacular, engine actually recommends premium and I've never quite understood this.
Over the last 11 tanks of gasoline I've done a modest experiment. I ran my first 5 tanks of gas on 87 octane, then performed a "washout" tank of premium, and then ran my next 5 tanks on 91 or 93 octane (whichever was available).
Regular octane average over 5 tanks = 24.1 mpg
Premium octane average over 5 tanks = 23.2 mpg
Two-sided t-test P value = 0.37 (not statistically significant)
Weather during this test was extremely similar, with most days around or below 32 F and some random warm days thrown in. This was all on winter fuel which is normally 10% ethanol. I kept track of the city/highway balance for each tank and this was similar across both groups as well.
So there isn't really a difference in fuel economy. What about driving feel? There does seem to be a very subtle difference in smoothness and "eagerness" of the engine when using premium. However, this isn't blinded or controlled in any manner so I can't be confident that this is a real difference.
Although the extra couple of bucks per tank isn't a huge deal, I'm sticking with 87 octane for now. The small difference in driving feel just doesn't seem worth the extra money with no improvement in fuel economy.
