Gas mileage variance

I've been doing a lot of trips of around 200 miles or so, and I am getting gas mileage varying from just under 24mpg to almost 28 mpg, with the same driving style. With the amount I drive, the 4mpg difference is noticable. I was at an almost constant just over 25mpg before I changed plugs and wires and did a Seafoam treatment. After the change and seafoam, I was at the 26-27 mpg range for a while. Now, one fill-up will barely make 24mpg, and another with the same driving conditions will make almost 28mpg. The change from a 26-27mpg to under 24-almost 28mpg has me wondering if something is wrong. Has anyone experienced this and corrected this?

By the way, no codes, and status shows EGR Not Ready.

Thanks for any help!

Ya got over 400,000 miles on it just be happy it is running. ;-)

Were all fill ups the same brand, grade and quality of gas? If you got an extra dose of corn in your gas, mileage will suffer.

If the change was during cold weather, perhaps your thermostat needs to be replaced.

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Originally Posted by EricJ

If the change was during cold weather, perhaps your thermostat needs to be replaced.

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I will try that. Thanks.

Ethanol content probably has the most to do with it.

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Originally Posted by VFR750

Ethanol content probably has the most to do with it.

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+1... I've noticed a measurable difference with corn squeezins, something on the order of 10-15%.

Also, any tiny variations in driving style can wreck your mileage or make you a champion. Several years ago in one road trip, I saw my mileage swing from a low in the mid 20s to a high over 35 mpg. Generally the same speed on the highway, but differences in winds and hills made HUGE differences.

For reference, around town my car gets about 24 mpg, and I don't drive it with care. On my last road trip, I averaged about 28 mpg between here and Hallett.

Agree with Rick. Driving style plays a role, along with E10+ content. I'm currently averaging 23.5 on my NA8 in the 6 weeks I've had it. That's almost exclusively city miles, <10% freeways/highways, mostly surface roads around Dallas proper. The NC averaged 22 in those same conditions, but pulled 29-30 on the open road.

Apart from the variables mentioned above, know that by volume ethanol contains about half the btu energy as gasoline.

So, expect your mpg to go down in proportion to the more ethanol percentage per gallon you use as fuel, with the same driving styles.
One up-side is that you gain a little more torque with more ethanol percentage used, also some increased compression.
Some may see it as a benefit that the combustion is cooler also.

Jay Johnson

Thanks for the advice. On most of the low mpg trips I used the same gas station. Basically, fill up at said gas station, drive a tank full, fill up, drive back, pay less for the return fill at higher per gallon prices. Maybe coincidence, but I will avoid them in the future, even though the ethanol content shouldn't have caused that big of a variance if the ethanol amount is reasonalby close to 10%.

I stil plan on changing the thermostat this weekend,

Thanks for the help.

Yeah, that doesn't sound like much variance in gas unless they filled the tank between your visits. Might be worth checking the tstat.