Ive been using 87 non ethanol lately..been noticing a spike in fuel mileage varying between 1.5 to 2.7 mpg depend on my driving. I could only get 16.5 was my highest for mixed driving, i was able to get 18.65. Ive been tracking this through the fillup app!

 

Might there be different formulations from winter to summer that might be responsible for this?

 

Im not sure, but ive noticed that when i do the same driving but mixed at least a half tank of non ethanol the mpg rises

 

Some of the driving varies...some is a crap ton of stop and go, for the lower numbers and the higher number are clearly more steady speed. You can see that after non ethanol fill ups the mpg does rise. Even sometimes my non ethanol mostly city driving coming on par with my 15%ethanol hwy drives

 

1- 2 mpg is meaningless. Weather changes, driving habits, traffic have that much effect.

Ethyl alcohol 75, 000 BTU/ gallon
Unleaded gas 105,000 BTU/gallon or so

Figure the change in BTU content from adding 10% alcohol. Trivial.

And PS. No one sells "100% gas no ethanol" That's a marketing scam.

The unleaded gas production process adds 3% ethyl alcohol to form an azetrope (3 way mixture of gas, water and alcohol) with the remaining water in the gas as its too expensive to distill it further to remove that tiny amount of water.

99% off this ethanol stuff is political hype.

Once I read a bulletin from the F.T.C. that said lower octane gas has more available energy than hi octane as it combusts more quickly. The down side is that it can allow detonation.

There may have been a tiny improvment based on lower octane.