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Variation in fuel economy

I have noticed my mpg has varied over the last 8 or 9 months in a peculiar pattern unrelated to the weather. Could the root cause be periodic variations in the chemistry of the gasoline that I use? I always buy the 87!

Are you buying an ethanol blend during winter months?


Describe the variation.

I don’t know anyone whose car gets the exact same mileage, day in, day out, over time. There’s always some variation.

Yes, it could be a variation in the gas. But more likely it’s a variation in the sensitivity of the gas pumps, when they “click off”. And then there’s those days when one finds it necessary to floor it to pass a semi going up an incline.

How much is the variation? How routinely do you check it? Can you describe the “peculiar pattern”?

Ethanol blend-this might be the root cause as you suggest. I see the energy density varies 25% between 34.8 MJ/L for strait gasoline (0% ethanol) and 25.2 MJ/L for E85 (85% ethanol). I always purchase the same grade, however the gasoline companies may vary the mix on their own, perhaps due to the gradual fluctuations in the price of ethanol.

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