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MPG calculator is WAYYY off

7K views 36 replies 21 participants last post by  BryST 
#1 ·
Sup everyone,

I tried to do a search but couldn't really find the answer to my question. Anyways, my issue is with the mpg calculator in the trip computer. First of all, let me say this is my first vehicle that has this feature. I'm aware that it most likely isn't perfectly accurate, but mine shows me really wonky readings. After a reset of the MPG, it will read fairly accurately for the first month or so. Then, its almost as if every time I shut the car off, and back on again, I gain 2mpg out of nowhere. At one point it was reading up to 45mpg which is obviously not true. I drive 100 miles a day, but its mostly on back country farm roads averaging around 50mph. I drive the car pretty easy most of the time on my commute. I just reset my MPG again the other day, and after the next tank of gas, was only showing about 26.5mpg, but by my calculations, I actually got 28.9mpg on that tank. It just boggles my mind how completely and utterly wrong this thing is. SO, am I the only one that has this issue, or is this fairly common? Its really not a huge deal because I know how to calculate mpg the old fashioned way, but if theres a fix for it, or a way to prevent it from happening, I'm all ears.

Anyone?
 
#4 ·
I have 6k+ since the last reset on my trip meter... reading 29.8MPG. I did a manual one a week ago and I got 29.6 I would say it is pretty darn accurate.

I do a manual one once in a while because some time I don't see the number change between tanks, even if I go drive it hard. then one day it will go from 29 to 27 mpg... drive for "normal" work week and it will go back to 29mpg... i could likely get better because it is all cruising highways but I mess it up by wanting to drive and have fun LOL

I have a 2015 with 30k miles.
 
#5 ·
I have 6k+ since the last reset on my trip meter... reading 29.8MPG. I did a manual one a week ago and I got 29.6 I would say it is pretty darn accurate.

I do a manual one once in a while because some time I don't see the number change between tanks, even if I go drive it hard. then one day it will go from 29 to 27 mpg... drive for "normal" work week and it will go back to 29mpg... i could likely get better because it is all cruising highways but I mess it up by wanting to drive and have fun LOL


I have a 2015 with 30k miles.
ok so yours actually works how its supposed to. I wonder why mine acts so weird? I have a 2016 with 8k miles.
 
#8 ·
I've never done mine manual but it doesn't bother me, tank is getting empty fill it and drive again until going empty, refill and repeat.
 
#11 ·
Can someone do a test for me? In the new Mustang, if you go into engineering mode you can adjust the MPG by editing the AFE value in that menu. To get to it, you hold the OK button on the steering wheel while you turn on the car. You cycle through the test screens with up and down and can edit some parameters with left, right and OK. If you have an AFE screen, you can edit it to be more accurate. I don't have an ST anymore to try with but if you have this screen, report back and I will tell you how to edit it. I know you can get into engineering mode on the Focus with the same method, just not sure you have all the same screens as I do.
 
#18 ·
I don't pay a lot of attention to my fuel economy because most of my driving is city miles and I have fun when I drive, so I'd just be disappointed anyway. Out of sight, out of mind.
 
#19 ·
I have a 15 F250, my 16 ST, the wife's Expedition and I reset the fuel mpg every single fillup and manually check mpg every fillup on all of them. On all 3 vehicles and every other Ford (10 Fusion, 12 Focus, 08 F150......) that I've had that I can think of have all been 1 mpg off +/- at each fillup. It's sort of a guide...
 
#20 ·
My 2012 SE was usually pessimistic by about 2 mpg, the ST is optimistic about the same. I reset at every fill up. FWIW, the SE averaged around 35 lifetime (cut short by a Camry at 204k miles) and the ST is averaging around 29 at 13k. Never had the numbers way off, probably the biggest delta in either car was less than 4 mpg.
 
#27 ·
I've seen the same thing - generally optimistic by 2mpg. I'm interested to hear about possibly adjusting the accuracy with the engineering mode. I've used a ScanGauge in a couple of cars and it dialed in pretty well, so I'd like to think the ST could be as well, as long as the engineering mode will allow it.
 
#22 ·
Avg 29? Wow

Must never be on boost when driving.....I am city/highway and lucky to get above 23-24. Oh well...

And I know what the original OP is talking about. If you watch the computer, it sure does wacky math!!!!!!! My moms 2012 Se focus doesn't jump all over the place in the mileage mode. Nothing like the 16 calculator
 
#24 ·
That's because the 2012 doesn't have an instantaneous MPG meter. The 2016 has both MPG average and MPG instantaneous. On the 2016 if you go into the 4 panel mode, the average MPG is upper right and instantaneous meter is bottom right. The instantanteous calculator was added to 2015 and above I believe, as my 2014 Focus SE did not have an instantaneous MPG reading, but rather a trip timer.

The instantaneous MPG meter lets you know how much your mileage is suffering when you give the car the beans.

I find the car is inaccurate from tank to tank by a couple of MPG but gets more accurate over several fill ups. Like any "average" the longer you measure, the less the number changes.
 
#25 ·
Mine has always been pretty accurate, within 0.2 MPG or so over time. The key is OVER TIME.

What I've found is that filling the tank is the inconsistent part. Sometimes when you fill the tank, the pump will shut off prematurely which makes the hand calculation for that tank much higher than reality. Other times, the pump will legitimately fill the tank. If the previous fillup's shutoff was premature, this tank's hand calculation will show much lower than reality, but the average between the two tanks will be pretty close to reality. I've never had this problem with any other vehicles, though even if I did, the small tank of the ST would exacerbate the issue. Since I haven't seen it with my other cars (though my boat does a similar thing), I assume it has to do with the ST's design, and possibly related to the capless design.

I haven't been keeping track of things manually now that my 16yo drives it and fills the tank pretty often, but over the first 30K miles or so, the on-board computer and the hand calculation were really close.
 
#26 ·
Nope mine is always off by at least 1mpg sometimes 2. The car will read "25.4" and I'll test it out to be 27.1 at the pump at time of refuel. I reset it every time I refuel. I might have to try resetting my car one of these days.
 
#28 ·
I didn't reset my trip2 for about 2k mi and it was giving me a 42mpg readout. I always hand calculate and only one tank during that 2k mi was just over 30mpg. Now that its been reset a few times it's reading 29.6mpg which I'm sure is still optimistic.
 
#29 ·
I'm only about ~1k miles into ownership and so far mine has been fairly accurate. I'm using the Fuel Buddy/Fuelio app's to track my fillups and mileage(as well as other costs/mods/etc) for ease of use and the trip computer is fairly close every time. Think it might be slightly pessimistic if anything, it usually hovers around 22-23mpg and then the app's tell me that my actual mileage is around 24-25mpg. Again, this is only over ~1k miles though so only about 4 fillups, we'll have to see how it goes over the next few thousand.
 
#30 ·
got around 26mpg out of my first two tanks, car only has 550 miles. Mostly Interstate driving miles(400) but the other 150 was around town boosting. I didn't do a manual check though, I will this tank.
 
#32 ·
The mileage calculator was working fine when I bought my '14. Had some body work done when someone backed into it. Body shop disconnected the battery, then the calculator went absolutely nuts. I could sit at an idle at a stop light & watch the mileage drop almost instantly from 15-20 to about 5. :serious: Gave up searching the web & bugging the dealer about it. Still reset it after every fill up, mostly city driving gets me about 20-22mpg, highway driving I've received about 33.4 on a long road trip, so apparently the calculator finally made up it's mind.
 
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