I have an 09 Subaru Outback (Bring it Im ready for the insults) and the MPG display seems weird. No matter what I do or how I drive I only get 15-17 MPG for the first 20 minutes of driving. Then after that its all over the board, the best being 27 MPG. What I dont understand is I can be coming off a straight mountain pass with a tail wind and it only shows 20 MPG, Then one day backing out opf my buddies driveway it shows 27 MPG. hOW EXACTLY DO THOSE THINGS WORK?

Computer and sensors

Computer is estimating based on speed, engine RPM and fuel flow (plus some other variables I am sure)

Probably like anything else, some versions are a lot more accurate than avg and some are on the low end of accurate.

I wouldn't trust it enough to let it strand me but can give you a decent idea in most cases

I used to calculate it on long trips to Florida each year. Id keep track paper/pencil at each filling and then calculate at the end of the trip...it was always within 1mpg of what my vehicle computer said. Back then, it was GM products, nowadays Ive smartened up a little and drive Toyota...havent done the calculations on them.

Subaru parked along a woods or river road I always think “ bird watcher, dog walker, gay men hook-up. Good cover for trapping

Older Tacoma parked along same road - trapping, fishing or hunting.

Just observations of a 60 year who has seen a lot.

On the Forester with boxer engine the EGR valve would
stick open causing engine fits. A simple cleaning helped that engine a lot.
A 3/8" drive swivel was needed to remove it.

I found a good Soob mechanic over at Slate Lick, Pa.

I have extra rims with new tires if interested.
The car will also get sold.

"They are all junk". Not just grey ones.

I wouldn't put too much faith in the "instant" mileage. But the average mileage seems to be pretty accurate every time I've checked it.

As far as the moment to moment MPG indicator, like you seem to be describing, I don't know. But the one that you are supposed to zero whenever you fill up and it will tell you what kind of fuel mileage you are getting that tank? I've checked it many times in my Dodge, and have also checked in several other brands, Toyota, GM, Ford, Jeep. EVERY SINGLE TIME I have done the math and checked, in EVERY BRAND, the computer has said it is getting better gas mileage than it actually is. Now if I do the math and sometimes it is higher, sometimes it is lower, depending on how much it is off by, I might say they aren't that accurate. But when they are wrong the same direction 100% of the time? The problem isn't that they inaccurate, it is that they are feeding you false information. The fact that it is that way across every brand I have checked, I find interesting.

Cat 2.

I know a girl that is getting a yearly check from
Kea er Kia because of the
"feeding you false information".
Class action or some such.
She says between that and the low cost
of operation she is happy.

They are junk too lol.

The calculations are only as good as the sensor information inputs, temp, humidity, air density, fuel flow, engine load, speed, etc., etc. if the sensors aren't operating at optimum levels the computer algorithm will be calculating with bad info ... Garbage in, garbage out!

Instantaneous mileage is really a distraction from your driving attention. In the early days it was a gauge indicating intake manifold vacuum.
Average milage will be close to calculations you have manually made; but you have to remember that your manual calculations assume the you consistently fill the vehicle fuel tank to the same level which isn't realistic as fuel level changes base on what grade the vehicle is setting on while being fueled, pump delivery pressure, humidity and atmospheric pressure, amount of evaporation over the time of fill up, error amount in the speedometer ( up to three percent ), actual tire pressure, and amount of tire wear ( related to actual tire circumference/revelation in calculation of driven distance).

If you're looking to compare your fuel milage to the EPA ratings, that will never happen as your driving and route specifics will never match the vary specific fuel course dynamometer program that the EPA ratings are tested on.

You won’t want to lol the gas mileage on those are terrible lol

I will reset the trip odometer when I fill up , then next fill up take the miles and divide by the gallons to get mpg average.

that was how we always did it before a computer fed us an answer that might be very accurate or might not.

LOF'NL. Good cover aye?

Hmmmnnn.....You got me thinking now.......How did you know it is Grey?

Any Soob that old in
Pa is primer grey lol.

Those KIA's may be cheap, with no resale value, but you can't hardly kill one....run it into the ground and go buy another one, lol