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I have to agree with Bartley that the mileage is best below 60 mph. Much better mileage at 50 mph than at 70. I can get 27 mpg averaging 45 mph with the AC running.

Also have to agree with Bounty Hunter that the displayed mileage is not to be trusted for long trips. I thought I was cool by not resetting the A odometer for 32,000 miles until I realized the mpg indication jumped around so it wasn't accurate.

Two big factors affecting mileage I have noticed,
1) outside air temp below maybe 25F your mileage goes down about 3 mpg this may be due in part to winter blend fuel contains fewer BTUs
2) cranking up the AC costs about 3 or 4 mpg
What I've found with my 18 is somewhat conflicting. Yes, under 60 mph is best, but not by much. If I travel at speeds less than 57 mph, the transmission hunts between 7th and 8th lowering the MPG. During the Winter, I drop 3-4 mpg pretty much across the board. But, running the AC has little to no effect. The 35 mpg trip I posted earlier was with the AC cranked the whole time.

I would say that idle time, along with "jack rabbit" acceleration, has the single most impact on lower MPG on a fully stock ECO. I've been known to shut my engine off at some of our 2+ minute traffic lights.
 

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How so? If you know that you drove X miles and it took Y gallons to fill up, you can figure out what your true MPG is
Whatcha quoting me for?

Had to dig deep to put context around what was quoted. Found that it was about 100 posts ago and think I was responding to using miles traveled on a tank vs mpg or something. Not disputing the how to calculate miles per gallon.

Anyhow, you are correct. Divide miles driven by approximate gallons used and you’ll get a fairly accurate mpg number.


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Discussion Starter #105
It's interesting that you averaged only 49mph for the entire trip. How much of the trip was spent at 80mph?
Running across SD in the 80 zones, we also took many older US highways traveling much lower speeds through small towns which helper the overall mpg.
 

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Took this pic this morning, getting 30 MPG over the past three weeks, over 500 miles of 100% city driving. I usually see about 35 mpg highway. If I hand calculate, it drops a little, but less than 1 mpg.
87527
 
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