Some posters have said that when they used premium/high octane, >87,
fuel in a car requiring only 87 octane, they got better MPGs. Others have
said the opposite.
Can both camps be correct? Apparently so.
Most sites that discuss premium vs, regular gas are silent on this matter,
But thankfully not
Click & Clack, The Tappet Brothers:
Finally, here's a nice irony: to increase gas' octane rating, companies add
ethanol, when they're mixing up a batch of premium fuel. Interestingly,
ethanol actually contains less energy than untreated gas, so the net result
from the ethanol component is a reduction in your MPG. Other premium
additives, however, have the reverse effect, and slightly increase your MPG.
So okay, overall premium provides a very slight net increase in MPG, but it's
so slight that we swear you won't notice the difference.
It would appear that just as when buying regular gas, finding premium
grade gas without ethanol could result in slightly higher MPGs.
FWIW, I'd bet
dollars to donuts that ethanol is the cheaper/est of the
additives used to achieve post-refined premium level octane readings.
From Wiki: Antiknock additives
Most countries have phased out leaded fuel. Different additives have replaced the lead compounds. The most popular additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).