“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Friday, July 28, 2006


It's been a very frustrating period. Over the last few weeks, my five tank moving average mileage has decreased by more than two miles per gallon. The troubles began with the brake job reported in an earlier post. The pads definitely dragged on the rotors for a few hundred miles after that - I could feel it in the behavior on coasting to a stop and, in the beginning, I could smell the burning pads. But these are no longer the case. The car rolls just like it always has, the highway instant miles per gallon seems normal, and yet my mileage has deteriorated. Why?

I got involved in an argument with a guy who has a blog called "mental radiation." (Note that that link is to the argument, I believe he actually maintains his blog at a wordpress site here.) The nature of the argument was over the extent to which the use of ethanol as a motor fuel eliminates the impact of driving on fossil fuel usage. As is common in internet arguments, neither of us was convinced. But in the process of our discussion, he looked at my blog and made several comments.

One of them was in response to my posting about my learning curve, in which I opined that the increasing trend in my mileage, as shown by the five and ten tank moving averages, indicated that I was learning to better maximize fuel efficiency. Bill Anderson, publisher of "mental radiation," was skeptical. His belief is that there are likely other reasons, e.g., different brands and different seasonal blends. I'm beginning to wonder.

I've bought Chevron, mostly from the same station, for the duration of the experiment. There have been perhaps three exceptions when I patronized Arco. The Arco purchases are not distinguishable on the graphs or in the raw data, but the downward trend from the last few weeks is obvious. I don't know if there has been a change in gasoline blends in June and July, certainly I haven't read of one. Maybe something's wrong with the vehicle. But even at that, why does my highway instant mileage still indicate what it did when my trend was upward?


Bill Anderson said...

I'd like to offer a possible explanation for instant not changing but overall average dropping. Lower quality gasoline has a larger impact at the worst possible moment: accelleration. If the fuel does have a lower energy quality it will perform significantly worse when you start up. This drop will not show on a cruising speed comparison.

Something I've learned about the computer controls may be of interest here as well. While the specifics are for GM LS series, I suspect they apply generally.

There are low and hgh octane "tables". Essentially these change how the fuel mixture and timing are set depending on how the fuel burns. Contrary to common sense the table is not reset and relearned when you fill the tank back up. As a result, if you get a bad octane fill up, by the time the computer figures it out, you are probably on a new tank of gas and probably with a better octane. Which can really hose things up performance-wise.

It is important here to bear in mind that filling stations do not change their octane stickers based on the batch of fuel they get. The octane rating is pretty much an estimate based on a sample from a similarly produced batch. The fluctuations in fuel quality are not generally published to the press. no conspiracy, just no interest.


King of the Road said...

Both very interesting comments and both seem like they could plausibly cause the symptoms I experienced.