I'm not sure what you're seeing, but I can tell you from personal experience with my vehicles that the Ethanol content makes a substantial difference in my mileage. 10% Ethanol (E10) caused me an immediate drop of almost 20 % in mileage when California outlawed MTBE and the so called "summer blend" gasolines were used year round. If you're seeing a marked difference try to track the Ethanol content and see if that correlates to your variations. The reason being the Ethanol has much less energy per unit volume than gasoline does. The BTU's (British Thermal Units) per pound are fairly similar but since alcohol weighs so much less per gallon than gasoline, the BTU's per gallon are quite a bit less if I remember my chemistry correctly.
I am getting better mpg too now that I switched from Shell to 7-11 Convenience Store brand gas. I agree 100% with Bates, some brands have less Ethanol than others so the gas with less Ethanol will provide better power and MPG due to the increased energy received from lower Ethanol. The Shell stations I used to use had signs at the pump saying it contains "Up To 10% Ethanol". 7-11 pumps say the gas has "Less Than 10% Ethanol". I think there are less detergents and Ethanol in 7-11 gas so it has more energy and I get better power and MPG from it. I will keep using 7-11 for a while, and see what my results are.
There is no power difference between different grades of gas that is great enough for you to notice.
The difference in energy content between pure gasoline and E10 is about 3%. The MPG difference will be similar to that. Since the octane rating of E10 is the same as gasoline of the same rating (premium, plus, or regular) there is no power difference between E10 and pure gas more than the energy content. The butt dyno simply does not have 3% accuracy. (E85 does have a much higher octane than premium gas, so it can give you more power if the computer is tuned for it.)
I have recorded E10 vs pure gas MPG differences right in the 2% range over a period of about 60 tanks. That was on an older car with a modern EFI system.
I have recorded much larger differences between summer and winter blends of gas. This year in OR the switch to summer gas was in late April and the switch back to winter was in mid-September. Over 24 tanks of this year's "summer blend" our Scion recorded a 31.3mpg average with a .86mpg stdev. The 41 tanks of "winter blend" prior to that (8/27-4/27) we recorded 29.2 mpg with a 1.3mpg st. dev (that included a 1700 mile road trip too). We've also recorded five tanks since the switch back to winter blend this year with an average of 28.3mpg. That is a 7% difference from last winter to this summer and a full 10% (3mpg) from this summer to this winter. The standard deviations were between 2% and 4.5% of average MPGs for each of those periods, so it is easy to pick out the switch.
Summer blends in general tend to give 5-10% better mileage than winter blends, simply because they contain less of the cheap volatile additives that have less energy in them. Summer gas has more gas in it, which is why it's more expensive and gives better mileage, in general. Winter gas is used when it's colder, so more of these additives are allowed because they are more volatile and give better cold-start behavior.
Bates' experience seems to be characteristic of about 10% of drivers, from what I've read. The fact that he is in Kalifornia may have something to do with it. Most other states do not exercise so much control over gas blends. But a 20% drop in mileage is not explainable by adding 10% ethanol, period, nor is it repeatable for the vast majority of cars. (Ethanol is actually MORE dense than gasoline, not the other way around, and the energy content by volume OR by mass is less than 4% by mass or by volume.) Since he has recorded all of his data, there's no reason to doubt his results, but I do think the culprit has to do with Cali blends rather than ethanol content. I hope we can hear more experiences, since Fuelly has a lot of people who have collected a lot of data on the subject.
BDC, I think you may be on to something with the Cali gas E-10 mileage drop being due to other additives rather than the Ethanol that are included due to AQMD mandates. The biggest drop correlated to the switch to Summer Blend prior to the outlawing of MTBE and then overall when MTBE was replaced with ethanol. I assumed by my mileage tracking that this was cause and effect. After reading your post I realize that there may have been other factors concurrent to the switch to summer blend and Ethanol replacing MTBE. Especially telling is your experience with better mileage using summer blend, I had a 10-15% reduction in mileage when So. Cal. switched from winter to summer gas. I have realized better mileage with out of state gas on road trips to Arizona, Nevada, Utah, and even Central and Northern California where the AQMD requirements are substantially different.
Thanks for the heads up, I'm going to take my wife's car to AZ this weekend and will try to track it then and compare it to previous trips there for reference. Great tip, I appreciate the insight.
I just don't understand why it would affect certain vehicles and not others. We've done a considerable amount of road tripping through all of California (about 5,000 miles recorded) on both summer and winter California gas and I never noticed a significant difference. All the tanks were on par with road trips on Oregon gas.
I've noticed each year since I moved 400 miles away from my family three years ago, that I lose about 1.5 to 2 mpg with winter gas. I go from 26 down to 24 on trips between LA and SF.
On one trip with the same gas, but a much colder day (35deg. vs. 65deg.) I lost another almost 2mpg due to the much denser air I was driving 75mph through. I couldn't believe it, barely got over 21 mpg. Of course the wind direction in the Central Valley can make a big difference too. It normally runs across or from the north so I regularly get 1/2mpg better heading to LA. With a nice tailwind, warm weather and summer gas I've had it up to 29.5mpg, but that was the perfect storm.
People need to keep in mind something else too that you don't think about. Warmer vs colder weather, plays a BIG part in your mileage, regardless of fuel. If you used the same fuel in both seasons, you will still see a difference in the MPG summer vs winter (depending on temperature changes). I think some of these variations you all might be seeing could relate partially or mostly to temperature for the differences, especially if your climate changes in temps a lot. I don't know the weather trends of California, but surely the northern areas see more differences especially. Just something to consider...
See, the thing is DTM, I notice an immediate 2-3mpg drop in mileage in the fall and the reverse in the spring. It is not temperature related. Since we go through a lot of gas in our Scion it's easy to pick out, and easy to corroborate with temperature.
Unfortunately, it is so hard to find solid information on the actual changes between summer and winter gas for any area that, besides empirical data that we gather from our fuel logs, it's very hard to determine the difference in energy content between the two.