ksimpson

I have my 96 C4S about a month now. I do a mixture of about 50/50 city/motorway driving. I've also been taking it fairly easy until I get used to the car and get new tyres fitted which should be done today (PZero Rossos).

I'm getting about 260 miles on just over 60 litres of fuel which I calculate to be about 19 mpg (UK Miles). However, the OBC tells me I'm getting 12.5 mpg. I also notice that when driving in the city, the average MPG falls very quickly to 11.5 and stays there until I do some motorway driving where it makes it's way slowly up to 12.5.

All other functions on the OBC appear to be working fine (except for the back light which has been discussed before).

Has anyone else experienced this problem?

Fred R. C4S

Ken,

Anything from the UK confuses me with kilometers/miles or liters/gallons/imperial gallons, so I don't know if this is causing your confusion. I assume you can make the necesary conversions from metric to US English. FWIW, 235/(l/100km)=miles/US gallons. Conversely, 235/(miles/US gallons)=l/100km.

My BMW 540i has an OBC that calculates fuel comsumption. One would think that the process is pretty straight forward. Assuming constant flow injectors, just multiply the injection duration times the flow rate and your get fuel consumed.
Divide the odometer by tis value and you have MPG. However, the BMW algorithm uses a constant in the claculation. It is set at the factory at 1000. By carfully manipulating the message board switch and the odometer reset switch, you can gain access to this variable and reset it. After several tank fulls, I was able to establish that the constand needed to be changed by 11%. I now get near exact MPG to what you claculate by hand using fuel pumped and miles driven.

I used to have a 964 with OBC. It generally was within 8-10% of my true mpg.

Sorry that I can't solve your problem.

JPS

Not sure about the one on yours, but often those are a running average. So if the previous owner was getting 10, and you are getting 20, your 20 will take awhile to average with a longer period of 10 mpg.

There likely is a place to reset, and then it will be more accurate for you.

Just a thought.

Pete Lech

With the Imperial gallon, I would think you would get higher miles per gallon than we do, and I average 24-26 on a trip, close to 20 banging around town. I don't think you have gone to different miles too?

Actually I never checked the accuracy of the OBC - the one in my 525i is pretty accurate, so I assumed this one was too. Think I will now.

rcwelch

ksimpson...The OBC in my car is pretty acurate as I check it all the time by simple math when I fill up...It proves to be right on. By pulling the stalk towards thr driver and holding it you can reset the mileage function each time you fill up...otherwise it will be an ongoing figure. Judging by the difference you are experiencing I would assume you need to reset the OBC mileage function.

ksimpson

I reset it after every fill up. At the moment it's telling me that I'm getting 11.5 mpg, but I have almost 250 miles on a full tank so far so it doesn't really add up. I'll mention it to my OPC at the next service.