The MPGs calculation my computer gives me is much too optimistic, does this mean my car is running much richer than the computer thinks it is? I guess another way to ask this is, is the ECU linked to the computer the MPG is calculated with or are they the same computer?

 

Interesting. Yesterday, I decided to trust my range left on my computer. It told me I had 7 miles left once I got to the gas station. I filled up the 13.2 gallon tank with 13.132 gallons.

I would say that's pretty spot on.

In terms of the MPG, I've yet to test it out.

 

moving this over to Beneath the Bonnet

 

test

 

Any ECU/computer gurus know anything about this?

 

>>Any ECU/computer gurus know anything about this?

I'm certainly not a "guru" but I have been trying to understand the relationship between the OBC, manuel computation of MPG and the ECU. This is what I think which may be way off base but ....
First, I think the OBC is very limited in what it can do. My guess is that it is receiving data from the ECU and then converting it to a user frienly format that we can read in the screen but it doesn't do much real computing.
Second, the MPG that the OBC reports are converted from Liters/100KM so you get some rather large jumps in the average that you wouldn't expect if it were averaging MPGs.
Third, I think the Liters/100Km information is being fed to the OBC from the ECU Fuel Map software NOT from actual fuel used. That is to say, depending upon the driving conditions the fuel map is programed to supply a certain Liters/100Km of fuel. Then a second(s) later another reading is sent to the ECU "fuel used" averaging computer and then every ? seconds transmited to the OBC. The OBC does its conversion to MPG and dispays it.
Fourth, this may have nothing to do with any of the above but the ECU reports a "full" tank as being 48 Liter no matter how much fuel is put in. I have taken a reading of remaining fuel, added fuel that would then total more than 48 Liters and the ECU will say only 48. This just leads me to the conclusion that I made in #3 above that actual fuel used has nothing to do with the reported MPG. I think the "fuel map" explanation may also account for the reported overstating of MPG by Cooper S but most Cooper are close.

 

I think you are correct in these things from what i have read on MCO and you do hereby qualify as OBC guru.
I will now call you OB wan edlfrey.
Thanks for your post. This makes sense. The OBC is not perfect and if we know it's limitations then that is good.

 

So with all the variances as to people's reports on accuracy of the ECU's computed MPG I think there might be something wrong. Some people are always low, some are always high, some are always right on the money as far as actual vs. computed MPG. What would cause this? A bad sensor? So if the sensor is inaccurate, how's the ECU to know what is the actual amount of fuel entering the system? So in actuallity my car might be running richer than it should?

 

Hello everybody...

I have a question regarding the OBC, but it has to do with the reporting units. Here in the US the OBC reports in english units, but I read somewhere in the owners manual, that the OBC was capable of reporting in metric units as well.
Does anybody know if it's possible to switch between units or it's something that has to be programmed by the dealer?

Thanks for your help!

Juan :smile:

 

After 2 1/2 months of watching and calculating the MPG, I've found that the computer is consistantly reporting 2.1 MPG low, i.e. it shows 33.1 avg. MPG and I calculate 35.2 avg. MPG.

 

Dominicminicoopers:
I don't think you need to worry about anything being wrong:
The OBC mileage is inaccurate, but that does not mean the engine is running too rich or too lean. The engine fuel mixture is adjusted based on mapped parameters, but also in a feedback loop using sensors in the system including oxygen sensor and air flow meter (usually a mass flow meter) etc... This results in very accurate adjustments based on the feedback loop, which take into account temperatures, humidity etc.. The OBC however - although this is only a guess - probably uses only an estimate of the metered fuel flow, not measured flow, maybe based on rpm and speed to fuel flow maps, or similar calculations, which would make them far less accurate than the fuel flow adjustment based on both maps and sensor feedback loops.
In addition, I have a nagging suspicion that the OBC is even more 'primitive' than that: I suspect the OBC only uses (o horror of horrors to those of us who dabble in measurement accuracy) the output of the fule tank filler gauge!!!

 

Hafid, I think you got lucky! Lots of people have posted here saying that they ran out of gas while their OBC range was 20 or 30 miles. Personally I don't let it go below 50 or so (and rarely even that low). I just don't trust it.

 

Me thinks you lot have way too much time on your hands.

Ken

 

>>After 2 1/2 months of watching and calculating the MPG, I've found that the computer is consistantly reporting 2.1 MPG low, i.e. it shows 33.1 avg. MPG and I calculate 35.2 avg. MPG.

At least yours is consistantly low. My is over by anywhere from 2.5 MPG to as high as 5.0 MPG too generous. You'd think it'd be consistant all the time, like yours, right?

 

>>I suspect the OBC only uses (o horror of horrors to those of us who dabble in measurement accuracy) the output of the fule tank filler gauge!!!

MG-BGT I think you're on to something here. I too have noticed if I max out my tank the first 100 miles there abouts, my computer estimates the MPG wayyyyy too high.

 

>>Hafid, I think you got lucky! Lots of people have posted here saying that they ran out of gas while their OBC range was 20 or 30 miles. Personally I don't let it go below 50 or so (and rarely even that low). I just don't trust it.

My fuel remaining seems to be pretty good. Only it adjusts itself to the needle like MG was saying. So when it reports 60 miles left to run, I know I can count on it being about 40 actual from that point. It self adjusts all the way down to the last drop, so the miles left actaully decrease MUCH quicker than the miles being driven.

 

>>Hello everybody...
>>
>>I have a question regarding the OBC, but it has to do with the reporting units. Here in the US the OBC reports in english units, but I read somewhere in the owners manual, that the OBC was capable of reporting in metric units as well.
>>Does anybody know if it's possible to switch between units or it's something that has to be programmed by the dealer?
>>
>>Thanks for your help!
>>
>>Juan :smile:

I am almost sure that the OBC is getting data from the ECU in Metric measure ie Liters/100Km for MPG. It then converts it to MPG. I'm also reasonable sure that the same OBC is used world wide so the OBC is capable of reporting in Metric, MPG(US Gal.) and MPG(UK Gal.). However, I am guessing that it "knows" which conversion to use by the ECUs Country Code. Again, I'm guessing but if you were to change the Country Code for the US (2) to the Country Code for Italy (?) you would get Metric readings. Long answer to say "I don't knowfor sure" but I don't think you can do it on your own.