I spent 30 years in fuel research at big ol' Texaco, Product Research, and we looked long and hard into everything about the various grades of fuel.
The higher grades of gaso had more aromatics than Regular making them more dense, more mass per unit volume. The carburetor cars showed better mpg with the higher grades because the carburetor system sucked the gaso over a hump to get it into the metering port. About the same mass of the more dense fuel was sucked over and that meant less volume. Hence fewer gallons to go the mile, throttle position staying constant. That led to all kinds of myths about better mpg with Premium gaso and was great as a selling point for the more profitable product to buyers who didn't need the increased octane rating. Back with leaded gaso, the lead increased the fuel density, and more lead was in the premium gaso to get the higher octane rating, hence contributing to the better mpg impression. The volatility of the fuel influenced startablilty and drivability at lower temperatures. The more volatile components are less dense and would tend to take away from an mpg rating.
With the expanded use of fuel injection, which meters the fuel by volume in a sealed system, fuel volatility was less to no influence and the direct effect on mpg went away. However, the same volume of the more dense fuel means more mass of fuel into each combustion event, hence more power. A driver looking for fuel economy and pussy footing the throttle pedal can get the desired performance/power with slightly less throttle thereby bringing a little better mpg.
A second however is that most drivers' rampage on the throttle overrides these effects and any improved mpg won't be seen. It takes very carefully controlled conditions to document these differences.
This consideration carries on to ethanol or any oxygenate. They are less dense than the gasoline hydrocarbon and additionally bring in less energy for their combustion event. Thus they are detrimental to fuel economy on a per gallon basis at equivalent power settings.
Edited by marshtex2, 28 October 2013 - 12:57 AM.