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Discussion Starter · #1 ·
Just curious about this....
When coasting down a moderate downhill (steep enough that the car will slowly gain speed by itself), if you shift from drive into neutral, have any of you noticed your instantaneous fuel mileage dropped significantly? When I shift back into drive, the instantaneous fuel mileage increases, if I shift back to neutral it drops. I haven't tried this on every hill around where I live, but I tried it on a few, and the same thing happens every time.

Has anyone else noticed this? It does not happen in my other cars, they stay pegged at 99.9 or whatever the maximum instantaneous MPG is that they'll show ( one of my Corvettes wouldn't go above 54.5 instantaneous MPG) regardless if I'm in drive or neutral. It's not a big deal, it's not really even that important, but I'm just curious if it's something other people have noticed
 

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To answer the first and last part, nope, haven’t noticed! But on my way to work in about an hour I’m going to try it!
 

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Not sure if I was able to coast on enough of a downhill or not. I didn't see the bar graph or instant mpg go down though. It came slightly off of 99.9 but not much.
 

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When coasting down a moderate downhill (steep enough that the car will slowly gain speed by itself), if you shift from drive into neutral, have any of you noticed your instantaneous fuel mileage dropped significantly?
I haven't tried it, but my guess is that the computer treats that situation as if you were sitting in your driveway idling. You're using fuel to keep the engine running, but the engine isn't contributing to any motion. So as far as the engine is concerned, it's giving you 0 miles for each gallon of gas it uses.
 

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Discussion Starter · #5 ·
I was kind of thinking that might be the situation the way the Ascent calculates mileage, and that certainly makes sense. I did try it in my f Pace this morning on the same stretch of road, and that went from 99.9 to 97.9mpg when I shifted into neutral, and back up to 99.9 when I put it back in drive, but the Subaru dropped by 30+ mpg in that same stretch. It's definitely not a big deal or an issue, I was just curious.

I wonder if there's an industry guideline/ SAE standard for how mileage is calculated by the in car computer? I did a quick check and didn't see anything...
 

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Idling always shows fuel economy to the full negative on the meter...it does the opposite while "coasting" in gear because the system feels that the efficiency is high due to lack of load. I suspect that when you "disconnect" the drivetrain, the fuel economy system thinks the vehicle isn't moving even though it might be.
 

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Your fuel economy meter is likely showing correct data. While coasting downhill, modern cars typically avoid injecting fuel into the combustion chamber. While in neutral, the car has to idle without the effect from the drive train being spun while coasting downhill, and therefore fuel is added to the combustion chamber to keep the engine running. Reference the two articles below from two different scientifically sound sources for details.

Bottom line: shifting to neutral while coasting downhill IS using more gas than leaving the car in gear.

https://www.popularmechanics.com/cars/hybrid-electric/a5977/coasting-in-neutral-fuel-economy/

https://www.roadandtrack.com/car-culture/a13026993/is-engine-braking-more-efficient-than-coasting/
 

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I just tried again. The bar graph and the instantaneous mpg are both maxed out while coasting in neutral
 

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^^^ yeah. Coasting in gear, the drivetrain is turning the engine over. Coasting in neutral, fuel is needed to do the same. IDK how the mileage display works under these conditions. It's software so who knows what it's doing...

Corollary: Some cars with big engines stop fuel to a number of cylinders when only low power is needed to improve mpg.
 

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Some cars with big engines stop fuel to a number of cylinders when only low power is needed to improve mpg.
Yes, multiple displacement has been in use for some time now, largely for V8 engines to help with highway fuel economy. The Grand Cherokee I previously drove had MDS on the Hemi and it permitted me to easily and consistently get 20-21 mpg highway on a 5300 lb vehicle while still having the horsepower for 7200 lbs of towing, etc. That system shuts off 4 cylinders when they are not needed. There are some multiple displacement systems that do variable numbers of cylinders depending on load.
 

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I tried mine down the two hills we live off of. The bar graph varied and increased as the speed descending down the hill increased. It never displayed 0 or even negative.
 
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