Winter mix and timing
 

gents:

been thinking over this winter mix question - since winter mix is more volatile than summer mix, would it make any sense to retard timing slightly? One other note here - the best mileage I have ever gotten was in april (minnesota). My thinking here is that the winter/summer blend was mixed together. all my cars are civics.

I don't believe that volatility is directly related to speed of combustion. If you have an early 90's or newer vehicle, your knock sensor/ECU will automatically retard timing to protect your engine from pre-ignition if it is sensed.

The best mpg can be usually be achieved with ignition timing advanced as much as possible without pre-ignition/spark knock.

Is high octane fuel more or less volatile than low octane fuel? Diesel fuel has higher octane than gasoline and also has a higher energy density and is much less volatile. There is much I do not know about octane ratings.
Regarding knock sensors and timing- for the knock sensor to kick in I would expect that a rather severe change in fuel burn rate would have to occur. The changes I am going to try are a few degrees, which I don't think would cause any action by knock sensors. I shall see.
Has anyone out there had an unexplained "great" tank of gas in mid season? I have recently. I went back over the numbers several times to check for errors, and could not find any - efficiency jumped several MPG for no good reason I could think of. Which leads me to think that there are significant variations in the gas mix regardless of the season.

Diesel fuel does not have higher octane. Actually, it's much lower. See Octane rating - Wikipedia, the free encyclopedia

Volatility isn't related to octane rating. Neither is energy content.

While I wouldn't be surprised at getting different formulations of fuel, there are so many far more likely explanations of random tanks having large fluctuations in fuel economy that it's really not on the list of things I'd bother looking at.