Bad gas? Sudden MPG drop.

I recently finished my very first tank, squeaking in over 500 miles, and with the Prius computer's estimate of 52mpg average.

I filled up at a brand-name gas station (Hess); the pump had a sign that said 10% ethanol, which I believe is the legal standard here in NY.

All of a sudden, despite my driving habits and routes remaining virtually the same, I'm barely squeezing out about 40-41 MPG ever since. This started immediately after filling up. For the first ~50 miles, I was getting about 36 mpg until I reset the thing in disgust (so that 40-41 is calculated without that first 50 or so miles).

Can I presume the gas is defective in some way, or is there another explanation?

I know that AC can make a difference but about 10 mpg? And though we have a heatwave here in NY, the mileage drop-off started before the heatwave hit, and without a change in my AC use. Would the car have been shipped with the tank full (and without an ethanol additive), or would it have been filled by the dealer, presumably with the 10%?

Any other guesses?

 

<div class='quotetop'>QUOTE(efusco @ Aug 6 2006, 09:35 PM) [snapback]298802[/snapback]</div>

Finally had the chance to check tire pressure, all four were at OEM specs. I upped that by 2psi each anyway.

A/C use has been consistent, both when I was getting 50 mpg and now that I'm averaging 42. The heat broke over the last few days and I've dropped A/C use (temp is set to 76 degrees), but mpg remains low (low compared to what I was getting, that is).

It was a sudden change right upon I refilling: dropping from 52+ mpg down to 42.

 

<div class='quotetop'>QUOTE(efusco @ Aug 8 2006, 10:26 AM) [snapback]299572[/snapback]</div>

500+ miles on the first tank, averaging 52mpg
About 800 miles on the second tank, averaging 42.6 mpg--and that is after resetting the thing to discount the first 75 miles (or whatever it was) averaging 36 mpg

It was a bright-line change: the day after the fill up, mpg plummeted. I'd averaged 52 mpg counting before I'd learned more economy tricks, so I'm sure I was averaging mid-50s for the last 100-200 miles. Then I filled up, and mpg dropped by almost 20 mpg until it crept back up to low/mid 40s.

 

<div class='quotetop'>QUOTE(sl7vk @ Aug 8 2006, 02:44 PM) [snapback]299709[/snapback]</div>

Yah, what Tripp said. I meant 800 miles total, 300 miles on the second tank. I was thinking "how many miles do I have on the car now?" and wrote down the answer.

I reset the MFD when I filled up, thus the low MPG reading on the second tank--the reading does not average in the first tanks mpg average.

 

Yep, it was the gas. I filled up at a different service station when 2 bars were left; at 67 miles on the new tank, my average MPG jumped from the mid-40s, which was the best I could average on tank #2 (or low 40s driving more "normally").


Warning: crappy camera phone pic to follow.

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