onboard mpg accuracy? - Jeep Compass Forum
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post #1 of 14 Old 05-11-2018, 12:36 PM Thread Starter
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onboard mpg accuracy?

First fillup
onboard trip meter said i was getting 35.6 mpg

it took 11.75gal over 389.4 miles (33.1mpg)

not much difference, should i chalk it up to dealer fillup vs my fillup?

fyi, i reset the trip meter, and its giving me 37.x mpg on my ride to work

i have 55 miles each way, mostly highway

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post #2 of 14 Old 05-11-2018, 01:46 PM
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Does the computer use an entirely different method to calculate the fuel economy? -> Yep.

Is the gas station fill method (miles driven / gallons put in) just a rough estimation with variance and consistency issues? -> Yep.

Then would it make sense that the computer and the hand math would pretty much always come up with slightly different numbers? -> Also Yep.

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post #3 of 14 Old 05-11-2018, 05:58 PM
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We use an app called Fuelly (Formerly Gas Cubby) that helps us track fuel fill ups and maintenance - love this little app really do. that said - I can see a difference between my Cherokee estimate and the log in Fuelly but as arudlang said - they calculate differently.

That said - there should not be a HUGE difference - so just monitor over time. I believe in another forum someone said the efficiency calc on board the Jeep had some fallacies based on the coasting calculation of use (like when estimated goes over 50mpg it starts to not add right)

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post #4 of 14 Old 05-11-2018, 06:01 PM Thread Starter
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yep. im using fuelly for the fillup calcs

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post #5 of 14 Old 05-11-2018, 07:17 PM
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The MPG computer accuracy seems to vary widely from Compass to Compass. I guess we are lucky with ours as doing the math a few times landed numbers that were well within what you would expect from slightly different fill levels. But others have had numbers that were anywhere from a little off to way off.

Which kind of blows my mind a little as the computer should be able to easily calculate the fuel flow through the injectors. A given pulse frequency at a given fuel pressure will flow amount of fuel then of course it knows the miles driven. So why these things wander so much I don't know. Maybe the manufacturers don't work so hard to improve accuracy.
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post #6 of 14 Old 05-12-2018, 04:18 AM
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The MPG computer accuracy seems to vary widely from Compass to Compass. I guess we are lucky with ours as doing the math a few times landed numbers that were well within what you would expect from slightly different fill levels. But others have had numbers that were anywhere from a little off to way off.

Which kind of blows my mind a little as the computer should be able to easily calculate the fuel flow through the injectors. A given pulse frequency at a given fuel pressure will flow amount of fuel then of course it knows the miles driven. So why these things wander so much I don't know. Maybe the manufacturers don't work so hard to improve accuracy.
Mine seems to be within rounding errors. So much so that I quit calculating it on mine. Pump variances make surprising swings in results, enough so that you can show two or three MPG off at times. Still, if you are consistently showing 10% less or higher variance with each fill, then I'd probably go with the calculator method.
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post #7 of 14 Old 05-22-2018, 04:24 PM Thread Starter
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so i filled up last night @ 5 miles from home
reset the trip meter
this morning when i left it said DTE said 387
after driving to work (55miles) it said 399 (trip meter said 39mpg btw)

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post #8 of 14 Old 05-22-2018, 06:51 PM
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Yeah the range on mine varies too, just like the fuel consumption it is constantly updating the estimated remaining miles based on how you are currently driving. If you are doing short trips to work day in and day out it will drop the estimate but after a number of miles on a highway at the beginning of a road trip it kinda catches on and ups the estimate if appropriate. So the estimated range you can think of pretty much as a slightly delayed estimate the car makes that is roughly valid IF you keep driving the way you are currently driving.

Its handy for me because I do pretty much the same drive 5 days a week so if I am not deviating from that much the range remaining ends up being pretty accurate for me and I know pretty good whether or not I am in a good position to wait for gas prices to drop or not.
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post #9 of 14 Old 05-22-2018, 07:01 PM Thread Starter
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yeah figured it was updating based on my driving.
just strange to see more miles to empty after driving 50 miles

but i do like the mpg on the highway, as thats most of my driving

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post #10 of 14 Old 06-22-2018, 12:37 PM Thread Starter
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4k miles in and it seems to finally be settling down
much better mpg than i had figured on


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post #11 of 14 Old 06-22-2018, 06:25 PM
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Only done one full tank so far and it seems to overestimate a little bit. But then all the cars I have owned have overestimated by about the same amount.

Computer said 50.9mpg on the first tank and my calculations say 48.5mpg. Not sure I filled back up to exactly the same point as one of the pumps was terrible so will see what the next tank is like as a comparison.

Either way I am happy with the mpg considering the car isnít run in yet.
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post #12 of 14 Old 06-25-2018, 11:57 AM
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Our Compass calculated and computer onboard display are generally within .2-.3 mpg of each other which I see as a very reasonable level of variation. We use Roadtrip which allows me to keep track of a variety of pieces of info relative to our cars.

We drive our 6 speed manual transmission conservatively and are seeing an average of 25.4 mpg in mixed city driving, most trips under 6 miles. So far we are pretty happy with the mileage.

This week we are taking our first real trip with the car so we should see how it does over extended highway miles. We will have a hitch mounted cargo bag so its unlikely to be the best it could be but comfort inside outwieghs hypermiling when in the car for 14 hours.

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post #13 of 14 Old 06-26-2018, 05:42 PM
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Mines pretty close to actual. Been tracking since purchase and it hasn't been much more than a few hundredths off.

http://www.fuelly.com/car/jeep/compass/2018/dawm/703280

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post #14 of 14 Old 06-26-2018, 06:25 PM
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Mine is accurate. 17-18 miles per gallon fuzzy math and 17-18 on the dash. Been like that since we owned it. Been telling FCA something is wrong between it burning oil and gasoline but no one wants to hear it.
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