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Discussion Starter · #1 ·
2015 3i Sport, 3k miles

the electronic fuel gauge is garbage!
when it's on E, I have between 2 and 4 gallons left. :surprise:

so I got to thinking about estimating the # of miles left by multiplying the avg mpg for the current fill up x fuel tank capacity (13.5 gallons.) minus # of miles driven.

so if avg mpg says 35 and tripometer = 410miles, then 35 x 13.5 = 472 - 410 = 62miles left.

how accurate is the avg mpg calculator?
 

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2015 3i Sport, 3k miles

the electronic fuel gauge is garbage!
when it's on E, I have between 2 and 4 gallons left. :surprise:

so I got to thinking about estimating the # of miles left by multiplying the avg mpg for the current fill up x fuel tank capacity (13.5 gallons.) minus # of miles driven.

so if avg mpg says 35 and tripometer = 410miles, then 35 x 13.5 = 472 - 410 = 62miles left.

how accurate is the avg mpg calculator?
Would be interested to know if the estimate idea worked for you...

Never noticed the calculation was off. Gonna have to start doing my own calculations to check with the next fill up. I've always trusted the guage, though I do know many cars will still have a gallon or so in the tank when on E. I prefer not to chance it and usually fill up between 1/4 and 1/8 of a tank left (per the gauge)
 

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In my car the AVG MPG is practically dead on. Twice after buying the car I did a manual calculation (miles driven/fuel added) and came to within .2 difference. Close enough for me.

Don
 

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My 2016 3i has proven to be a bit optimistic, estimating about 1 mpg more than actual, but that wouldn't affect your calculation by very much unless you're really trying to stretch out every last mile.

And, with all due respect, I wouldn't really call the gage garbage ... every car (that I can recall, anyway) that I've taken to empty on the gage has had at least 2-3 gallons left in the tank. Seems like a responsible car design feature to me.
 

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Discussion Starter · #7 · (Edited)
And, with all due respect, I wouldn't really call the gage garbage ... every car (that I can recall, anyway) that I've taken to empty on the gage has had at least 2-3 gallons left in the tank. Seems like a responsible car design feature to me.
yes, cars are designed to have some gas left after it reaches E.
but in my Mazda3i, it's inconsistent as to how much gas is left.

some full ups it's 2 gallons. others it's 4gallons!
4 x 40mpg = 160miles

that's 80miles more I could have driven b4 wasting my time filling up.
multiple that by 200k miles over the life of the car and that's ALOT of time saved.

but why is the electronic fuel gauge inconsistent? ???
 

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I'm at 9K+ on my 2015 sGT HB, and the car-computed average says 35; my observed average is 35.8. That's only a 2.2% discrepancy.
 

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yes, cars are designed to have some gas left after it reaches E.
but in my Mazda3i, it's inconsistent as to how much gas is left.

some full ups it's 2 gallons. others it's 4gallons!
4 x 40mpg = 160miles

that's 80miles more I could have driven b4 wasting my time filling up.
multiple that by 200k miles over the life of the car and that's ALOT of time saved.

but why is the electronic fuel gauge inconsistent? ???
I haven't noticed that much of an inconsistency. If I had to guess, I would say that something is confusing the mpg estimating routine sometimes (e.g., variations in conditions, etc.), and that is causing it to miscalculate the miles to "empty".
 

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Mine is pretty accurate as well. Only thing off is when filling at different stations, it'll sometimes stop at 10'ish gallons or 11'ish and one station it stopped at under 9 gallons. I was thinking it might be due to their floor not flat and car is in a slight angle or something, idk lol I usually try to fill up at the same station, same pump and it's pretty consistent like that.
 

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Short answer: very very accurate.

Long answer (read at your own risk, bring coffee):

I have 27K miles of manual logs so far that I keep in Excel to analyze the car. I measure the mileage manually each tank and I also record what the trip computer metrics are for each tank so I can run comparisons. And when I say metrics I mean ALL of them: trip odometer miles, actual odometer miles, mpg for that tank as recorded by the car, DTE when fueling, avg speed for that tank, whether or not the fuel light came on and if so what mileage was the trip odometer at when it did come on, what was the DTE reading when the fuel light came on, and if I let the car go to 0 miles on the DTE readout at what mileage on the trip odometer did it occur, gallons used, price paid and price per gallon.

I can tell you that based on my model ('15 2.5sGT hatch, manual trans), with my driving style/commute, etc., I have never had any individual tank have an error of more than 0.9 mpg in either direction. And most times it's fairly close to accurate. In fact if there's a wide deviation on a particular tank, I'll often notice the next tank tends to correct the other way. That indicates the fuel pumps at the stations themselves aren't precise in their cutoff methodology (despite my very precise way of fueling my car -- fill it, when it cuts off, count to 60, then top it off 'til one more click) and generally supports the accuracy of the trip computer.

Over the life of the car so far my real world mileage -- total miles driven since delivery divided by total gallons used since delivery -- is actually only 0.2 mpg higher of the calculated mileage by the car's trip computer (and I do it the precise long form way -- that I won't get into here -- not just the average of the car's tanks recorded each time).

I'm averaging about 35 mpg lifetime (probably a little less since we're in winter and it falls in winter) so that's a margin of error of about 0.6% which is pretty solid.

So yeah, it's pretty accurate based on a sample size of me. ;)

You should see all of the other metrics I run on this thing. :p
 

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(despite my very precise way of fueling my car -- fill it, when it cuts off, count to 60, then top it off 'til one more click) and generally supports the accuracy of the trip computer.
If you don't mind my asking - when you use the above method of fueling up, how much gas goes in on the second go-round (after waiting a minute for the tank to settle)
 

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If you don't mind my asking - when you use the above method of fueling up, how much gas goes in on the second go-round (after waiting a minute for the tank to settle)
Sure -- happy to answer (nice avatar, by the way, I'm a Seahawks fan too despite my geographic location -- the real kind who has pics of himself as a little kid with 'Hawks jackets, pennants and toys and can name all of the players back into the early 1980s when I started my football awareness, not the fake 'hey they just won the Super Bowl is there room for me on the bandwagon' kind).

;)

Just this morning I filled it and it took a hair under 0.4 additional gallons on the second go-round. That's as typical as it gets as it's almost always between 0.3 and 0.4 additional gallons, weighted toward the top end of that range. I've had a few outliers approach 0.5 but that's what pump variability (e.g.: how aggressive is the shut-off, how fast does the petrol flow, etc.) sometimes gets you.
 

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Thank you for the answer! (and sorry for taking so long to reply - works been running me every which way this week)... sounds like at least when the gas station is empty, it is worth waiting a bit to let the fuel settle.

And even though I've always lived in the East Coast Washington area, I've been a Seahawk fan since the Baltimore Colts left the region - going back to the Jim Zorn, Steve Largent, Curt Warner, Ground Chuck days... and have a Largent mid-80's royal blue AFC West replica that still fits 30+ years later. lol. I hear ya on the bandwagon fans - a few are decent, but had to laugh when some were saying that this past season was the worst ever... *smh @ them*.
 

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On my 2010 it's usually well within 1 mpg.

As for the gauge, not sure. I know they put a buffer in there due to people not paying attention or just being a bit "slow" paying attention to their gauges. Keeps the fuel pump from overheating as some are cool by the gas, also keeps them from sucking up whatever junk settles in the tank after lots of fill-ups.
 

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I sit in traffic quite a bit (my average speed is 18 mph over 10,000 miles) and the digital average plummets when idling. In the city it is way off, but on the highway it is within 1/2 mpg. Next time I fill up I will do the math and get a solid difference.
 
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