I should justify first.

We all know that in most places in the US these days our gasoline is laced with 10% ethanol. In fact there is discussion now that the EPA allow the refineries to up that to 15%. (Be forewarned and aware people.) And we should know that alcohol of any kind, carries with it its own oxygen molecule causing a modern electronically controlled fuel injection system to richen up the mixture to compensate.

My normal drive is 16.2-16.4 miles one way, depending on the route I take, or a total of 32.4-32.8 miles/day.
I am one of those **** retentive guys that takes a receipt every time I fill up, and I rarely do anything BUT fill up. I check my mileage religiously and on the spot.

Lately I have noticed a decline in my fuel mileage by between 2 and 3 MPG and am trying to figure out what might be causing it. I used to get between 24-26mpg during the summer months when the A/C is running all the time, and 27-29mpg when the A/C is off. In fact they were even on the rise after the car hit 35k miles!! My best was 30mpg in normal traffic during one tank full. My most recent "best" was when we took a day-trip to San Antonio and I managed only 27.37 for the tank. THAT's TERRIBLE compared to what I was used to.

My last 9 fill ups show an average of 25.52mpg, where as last year during this same period the average was 27.89mpg.
I highly doubt my driving habits or the situations I drive in have changed that radically during this time.

Has anyone else (that regularly checks and documents their mileage) noticed a change, or this radical a change in mileage?

 

On a different board I've seen any number of similar comments, and IIRC, the problem is usually traced to a sensor (oxygen sensor I think) beginning its final miles of usefulness.

FWIW, a move from 10% ethanol to 15% can, at its maximum, only drop mileage a little less than 2%. Consider the following:

BTUs in 1 gallon of E000: 114,000 -- Reduction: 00.00%
BTUs in 1 gallon of E100: 076,100 -- Reduction: 33.25%
BTUs in 1 gallon of E010: 110,210 -- Reduction: 03.32%
BTUs in 1 gallon of E015: 108,315 -- Reduction: 04.99%

 

My mileage has dipped in the last two tanks as well, but we've been on winter mixture up north. Can't say that should be the problem for you, Rich.

 

I guess I should add, another factor I just thought of (and forgot to mention before) is that I run Shell gas almost exclusively and up to now been very happy with the quality of the fuel.
But it occurs to me that Shell engineers have been "playing around" by adding nitrogen into the mix. (well documented in their TV ads) and wonder if this is the difference?

 

That's a pretty big variance for one supplier, it must be said. I usually go to a few different stations periodically to see if they've changed anything. The Malibu loves Gulf, but gets crap mileage most of the time; it hates Sunoco and gets poor mileage ALL of the time; it likes Mobil1 95% of tanks it gets.

Just food for thought. Switch to a new station and see what happens.

 

Sadly, we have none of those stations/brands here. I grew up w/them and know them well, only not here.
My first job (16) was at a Mobil station (though no longer) and I still visit its owner whenever we go home for a visit. He's in his '70's now and semi-retired. Among the best street fuels available back then. As was Sunoco.
I grew up in a small town that had 3, count 'em 3 gas/service stations. An Atlantic, later Arco. A Gulf station owned by a little old man even back then. Its now a bank branch and flower shop. And the Mobil station previously mentioned, first with lead free gasoline, though not magnesium free. We called it "clear" gas at that time and only the super was lead free.

 

i just can't believe that your mileage is so consistent. i check my mileage constantly and i do between 28-35 mpg no matter the situation. i have gotten very consistent long term averages at about 32. i have never had two tanks in a row identical though.

 

Mileage in '04-'06 w-A/C was between 24-25... disappointing. After '08 it went up to between 26-27 during the summer months.
The BEST I could do when the car was new (no A/C) was right around 28. 26-27 was more average. Then in '08 it went up to averaging near 29.
I have not changed jobs or locations. I mostly use the car for back and forth to work and have two very similar route options, only 0.2 difference in mileage.
What can I say? Foot in, or foot out, short-shifting or winding her out that's the way its been?
Sometimes when I know I've been driving "spiritedly" all week long, I am surprised that the mileage had remained more or less the same as before, and occassionally even better than my previous average!! I know it doesn't make sense, but that's what I've calculated.

 

when i hear something like 30 miles per gallon and how you fill the tank,reset trip,drive to 1/4 tank and fill up again and check miles and do some math.wouldnt it be cool to take like a corvette with no gas,put ONE gallon in,get on the autobahn and just floor it till your out of gas and the car rolls to a stop and check the miles.how many miles to ONE gallon would you think? soemthing ive always thought of.yes its unfeezable but maybe in a laboratory or something. im thinking the car would roll for about 3 to 5 or 6 miles after the engine stops, going 200 mph.think im crazy? so that would be about what ? 35 mpg in a vette?