MPG – computer vs calculated - 2017 Ford Fusion V6 Sport Forum
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post #1 of 33 (permalink) Old 12-30-2018, 09:54 PM Thread Starter
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MPG – computer vs calculated

A random discussion for everyone. I know it’s generally accepted computer calculations are incorrect, but does anyone know why the computer per trip MPG calculation is terrible with the FFS? Is this a general Ford Fusion issue, did Ford not modify the calculation for the FFS differences or is this just an issue with mine, such as outdated firmware? Mine is a stock May 2017 build but I don’t know what IDS version number is installed for the various modules. I can’t assume the dealer maintained updates prior to selling.

I log this information but of course I didn’t buy the car for impressive MPG. I don’t care for underpowered average sedans, so this isn’t a complaint about low MPG. This of course is griping about a tiny feature that doesn’t impact the performance of the vehicle but if a feature exists, it should be reasonably accurate and consistent. I can drive the car easy or hard. I find the computer vs hand calc closer if I have a little fun with the car a few times throughout a tank of fuel.

Here are some previous fills, most recent to oldest. I reset trip 1 after each refill and run premium (93).
This most recent was a mix of city and 45-55 MPH roads without any goofing off and it’s the largest difference I’ve seen yet. Due to heavy Christmas traffic, average speed of 27.86 MPH though.
Considering how bad it is, the lifetime average isn’t too off at 17.7 MPG (computer 19.2).

16.5 MPG (computer 20.0) (what?)
18.7 MPG (computer 19.9)
17.6 MPG (computer 18.8)
17.8 MPG (computer 19.9)
18.3 MPG (computer 20.5)
18.1 MPG (computer 20.0)
16.9 MPG (computer 19.1)
24.0 MPG (computer 25.0)
17.5 MPG (computer 18.8)
17.5 MPG (computer 19.0)
16.7 MPG (computer 18.8)
16.7 MPG (computer 18.2)

My F-250 had an average of 99.7% accuracy with OEM tires (275/65-18) when new and is 0.2 MPG low (computer) with slightly larger tires (295/70-18) now so I know Ford knows how to dial in these computer reported calculations on some vehicles (computer is configured to 34.0” vs tire is 34.3”). My previous vehicles didn’t have a MPG screen and I don't pay much attention to rentals when traveling to know if other brands are better or worse.

2017 Fusion Sport 2.7L 401A (06/17)
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post #2 of 33 (permalink) Old 02-13-2019, 08:10 AM
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Mine usually is 1-1.5 MPG lower than what computer says.
My dodge diesel was the same way. My 14 mustang was 1 MPG
higher real world, vs computer (which was a nice surprise).

I've had some anomalies, but never pushing 4 MPG difference.
Mostly yours seems about what I get.
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post #3 of 33 (permalink) Old 02-13-2019, 07:05 PM
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Originally Posted by kper05 View Post

16.5 MPG (computer 20.0) (what?)
18.7 MPG (computer 19.9)
17.6 MPG (computer 18.8)
17.8 MPG (computer 19.9)
18.3 MPG (computer 20.5)
18.1 MPG (computer 20.0)
16.9 MPG (computer 19.1)
24.0 MPG (computer 25.0)
17.5 MPG (computer 18.8)
17.5 MPG (computer 19.0)
16.7 MPG (computer 18.8)
16.7 MPG (computer 18.2)

My F-250 had an average of 99.7% accuracy ...
Excellent work.

Seems like the FS's computer accuracy is 92.2%, lol.

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post #4 of 33 (permalink) Old 02-13-2019, 07:57 PM
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Mine usually shows 1mpg better than actual...

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post #5 of 33 (permalink) Old 02-13-2019, 07:58 PM Thread Starter
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Since this post I accidentally used trip B for two tanks in mid January. One was spot on at 18.9 both computer and hand calc. I thought I was on to something (programming error with screen A). However, the second one erased that idea at 19.2 computer and 17.7 hand calc.

I can't explain the 16.5/20 but that is the only time and was the 20th tank.

So far lifetime average is 17.7 hand calc on 93.

2017 Fusion Sport 2.7L 401A (06/17)
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post #6 of 33 (permalink) Old 02-14-2019, 06:44 PM
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With only a few tanks on our FFS, I’ve observed about a 0.25 MPG difference, with the computer being optimistic. I do only fill the tank to the initial pump shutoff, and do not try to “top her off”.
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post #7 of 33 (permalink) Old 02-14-2019, 07:30 PM
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Mine had always been about 1-1.5 high on the computer.

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post #8 of 33 (permalink) Old 02-14-2019, 08:27 PM
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I found mine was pretty accurate over a full tank. Accuracy decreased a bit on half tank fill ups.

How much do you have the car warming up on remote start? This would impact your hand calculations.

Without having actually tested the theory, it is possible that the computer is measuring in metric and the error comes about in the conversion to imperial. Fusion is originally European. It is possible all of the sensors were calibrated in SI (metric) and the conversion math doesn't have enough decimal places for accuracy.

Had a similar issue with measuring flow rates of pumps in a previous life and ended up discovering the number of decimal places in the flow meter to convert to US units was insufficient and impacted the accuracy.

This can present as an issue as the flow meter is likely measuring something like millilitres per second so the multiplication needs around 8 to 10 decimal places to go from metric to gallons with a degree of accuracy. Betting the software doesn't go beyond 6 decimal places. Likely 5, which is what we found out from the flow meter manufacturer.

Worth noting that the sampling rate and averaging by the computer is fairly archaic. I've been in friends beamers and audis and their fuel efficiency is shown in real time. On ours the averaging calculation is so slow that it takes about 15 minutes or so before the number plateaus out to something close to what you are consuming after a reset. Even at constant speed. Do a reset while you have cruise on to see what I mean.

Just a theory and some babbling out loud.
You've got me intrigued enough to start keeping the gas receipts again and resetting the trip computer. I am interested to see whether it's still close. Just realized that I work in litres/100km which would mean I wouldn't see the difference (error) as much. I'll do my calc's in both units as I might have the same error, just haven't noticed.
Thanks for posting your numbers.

Just did the math. Conversion error is only going to be about 0.1% in our case. Bit of a waste of the readers time-sorry

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Last edited by Fozzy; 02-14-2019 at 08:48 PM. Reason: Appology
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post #9 of 33 (permalink) Old 02-14-2019, 09:39 PM Thread Starter
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No issue with out loud babbling, I do it to. Just good (random) discussions.

I remote start I'll guess 60% of the time while approaching the car. 1.5 MPG would be a decent amount of idling though. However, that is a good point because I'm not sure the computer is logging the trip A/B info until accessory power up. I have not considered this.

< 30 seconds to finish approaching, +30-60 seconds to get situated, seat belt and maybe open Spotify when it doesn't start playing half the time on its own.
I've started at least 12 times over three days this week and at least six I know were push button. No idea for the rest of the tank and need to fuel tomorrow.
I now need to avoid remote starting for a few tanks but I don't know if I can achieve that... LOL.

Does anyone know the series of buttons to press between KOEO to KOER to get the dash into engineering mode? I want to see my running vs idle hours now.
I thought it was hold right OK at start. I tried from off and from accessory mode. I'll check the university of Google and retry later.
I have no reason for keeping up with MPG details other than I just like the data.

Edit: Holding left OK button at start is engineering mode. No idle hours though.

2017 Fusion Sport 2.7L 401A (06/17)

Last edited by kper05; 02-14-2019 at 10:38 PM. Reason: Left OK button... not right
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post #10 of 33 (permalink) Old 02-14-2019, 09:59 PM
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I just got gas today and checked this. Computer said 23.3, actual mileage was 21.9 mpg.

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