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Discussion Starter · #1 ·
Just got back from a weekend trip: 388 miles, average 23.6 mpg including mostly highway with some city driving.
On the way there, about 170 miles, it averaged 26.6 mpg driving 75/80 in the highway, no stops. During the trip I kept the instantaneous mpg visible and it varied from 22 to 30 mpg, and the average resulted 26.6mpg (I'm happy with that).
On the way back, exact same road, completely flat BTW, elevation difference is roughly 30 ft on 170 miles, coming back should be going down, so the same or better mpg was expected.
I did not want to reset the average but by looking at the instantaneous mpg I saw it mostly vary from 17 mpg to 25 mpg, and the average was not moving from 23.0.
About half trip I decided to set the gearbox in manual, 8th to see if that improved the mpg. It did not move the average either.
I was about 50 miles from destination when the engine stuttered (not too bad but very noticeable). 2 times first with 1 second between them, then a third time a few seconds later. I read somewhere in the forum about these stutters but never happened to mine until now (car has about 1300 miles).
I didn't think much at the time but 5 minutes later I noticed the mpg was now oscillating between 25 and 35 mpg, I was still driving on the same flat road at 75 mph, nothing else had changed. The improvement was noticeable, I felt I had to go lighter on the throttle to stay at 75 mph, so I don't think it's a bad measurement of the mpg, it really was sipping less gas (somehow?).

The mpg improved so much that it was enough to improve the average from 23.0 to 23.6 mpg (in the last 40 miles alone!), so it wasn't just my personal perception. My way back I drove 130 miles with the average nailed at 22.9/23.0 and in 40 miles it went up so much? I think it's bizarre enough to report it. My wild guess is perhaps some dirt got through the injectors?


I'd like to hear possible explanations. I didn't believe other posters about poor mileage before, but now I can bear witness: my car had highway mpg around the advertised 27 since I bought it. This is the first time I notice the mpg swing so wildly.
 

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Did you leave your wife and kids there? Weight difference could affect mpg. Did you press auto on the heat control knob? That could have turned on the AC which will blend with heat but use several mpg.
 

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I had a similar experience. I mostly drove on the way and on the way back my wife mostly drove. There was a noticeable drop in MPG on the way back. :tango_face_surprise:dunno:
 

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Discussion Starter · #6 ·
Headwind versus tailwind?
Perhaps between the way there and the way back, but wasn't particularly windy either day, and I didn't cross any storm front or anything that could suddenly change the wind direction, but can't be ruled out. I didn't stop the car the whole way back so I couldn't say it for sure.
Let's keep this as unlikely but entirely possible.
 

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Discussion Starter · #7 ·
Did you leave your wife and kids there? Weight difference could affect mpg. Did you press auto on the heat control knob? That could have turned on the AC which will blend with heat but use several mpg.
I counted, all kids made it back, wife and mother in law also safe and salve. Same weight in bags, same everything. Temperature was the same, about 84 both ways, AC was blasting at 72 all the way (auto mode), no changes there (temperature changed a little during the trip but not by much since it's less than 3hrs). I think the mpg jump was too much of a change to be only due to a couple degrees change in temperature.
Perhaps the right way to test it would be doing a trip with AC on and one with AC off to see what happens with mpg. I don't volunteer though, I like my AC working always (they could remove the heat option in the car and I wouldn't care but don't mess with the AC).

So AC may have contributed a small amount, but it's hard to blame AC for this with 1 or 2 degrees difference only.
 

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Discussion Starter · #8 ·
Keep in mind that this time of the year you may be randomly purchasing a winter gasoline formulation which will give noticeably less gas mileage.
If I had bad mpg on the entire trip back, that would've been my guess as well. Even though I put the same Chevron 93 with Techron that I usually use, it was an unknown gas station in an unfamiliar town. But this doesn't explain the change while driving. When I filled up there was less than 1/4 tank, so it was pretty much the new gas all the way.
 

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Discussion Starter · #9 ·
Perhaps between the way there and the way back, but wasn't particularly windy either day, and I didn't cross any storm front or anything that could suddenly change the wind direction, but can't be ruled out. I didn't stop the car the whole way back so I couldn't say it for sure.
Let's keep this as unlikely but entirely possible.
Looking at weather maps, a plausible explanation would be that wind was almost exactly in opposite direction to the car (car was going SE). I noticed the last 50 miles of the trip have an area where the road turns quite a few degrees more towards E then compensates turning more towards S. So if the wind was exactly opposite to the car for the first part of the trip, it would've been few degrees off left and right in the last part of the trip. I didnt feel strong winds, but perhaps 10 mph wind or so is enough to bring down the mpg drastically. Can a few degrees make that much difference? I'm inclined to think so, because of the square front grille. When hit directly from the front it has to be bad (air is hitting a wall), but as soon as the wind comes from a corner it's easier to split the flow. So I'll call this mystery resolved. Thanks for the tip!
 

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Running the AC knocks off about 3 mpg on mine. I did notice that is sucks gas when it is cold. Until it warms up, I can almost see my gas gauge drop. Seriously, I did have 150 miles on a tank and had averaged 20.2 mpg since filling it up. I started it one morning and in the first mile, the mpg dropped to 18.8.



Acceleration will be costly too. I haven't pushed mine all out yet but just merging on a freeway can cause a drop in mpg.


It may be time to just enjoy the car and fill it up when the tank gets low. At current gas prices, economy is less of an issue.
 

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I'm mining the data to do the MPG evaluation. After two tanks of gas, my hand calculation have shown to be 1.2MPG lower than the computer. I believe the hand calculation is more accurate than the computer, simply because the data is originated from the source (calibrated/QC'ed/regulated gas dispenser).

So far I'm averaging 21.5MPG. In march, I will know exactly the average MPG of my Limited.
 

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I just took a 20 mile road trip all highway, speeds,70-75mph. Weather 50 degrees winds 5-10 mph. 29.7 AVEARAGE
 
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