# Variation in fuel economy

**URL:** <https://community.cartalk.com/t/variation-in-fuel-economy/52463>\
**Category:** General Discussion\
**Tags:** neon, dodge, fuel-economy\
**Created:** [July 25, 2010, 10:17pm UTC](https://community.cartalk.com/t/variation-in-fuel-economy/52463 "2010-07-25T22:17:35Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![witbier](https://avatars.discourse-cdn.com/v4/letter/w/d26b3c/32.png) [@witbier](https://community.cartalk.com/u/witbier)\
**Post date:** [July 25, 2010, 10:17pm UTC](https://community.cartalk.com/t/variation-in-fuel-economy/52463/1 "2010-07-25T22:17:35Z")

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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!

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**Author:** ![twotone](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@twotone](https://community.cartalk.com/u/twotone)\
**Post date:** [July 25, 2010, 10:24pm UTC](https://community.cartalk.com/t/variation-in-fuel-economy/52463/2 "2010-07-25T22:24:35Z")

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Are you buying an ethanol blend during winter months?

Twotone

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**Author:** ![mcparadise](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mcparadise/32/32_2.png) [@mcparadise](https://community.cartalk.com/u/mcparadise)\
**Post date:** [July 27, 2010, 6:20pm UTC](https://community.cartalk.com/t/variation-in-fuel-economy/52463/3 "2010-07-27T18:20:31Z")

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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.

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**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\
**Post date:** [July 27, 2010, 9:44pm UTC](https://community.cartalk.com/t/variation-in-fuel-economy/52463/4 "2010-07-27T21:44:28Z")

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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”?

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**Author:** ![witbier](https://avatars.discourse-cdn.com/v4/letter/w/d26b3c/32.png) [@witbier](https://community.cartalk.com/u/witbier)\
**Post date:** [July 28, 2010, 2:58am UTC](https://community.cartalk.com/t/variation-in-fuel-economy/52463/5 "2010-07-28T02:58:12Z")

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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.

(data file attached)