Trip computer and ave consumption accuracy

[frikkieh]

Fellow fanatics :wave:,
My car gives an avarage fuel consumption of say; 9.4l/100km, but if I fill the tank and I do the calculations, I get a better consumption - evey time. The prediction for distance to empty is also from Mars - quite some time ago,
I had 74km left and relying on German technology the car just stopped on the way to the filling station. :dropjaw:
Well, a 3rd class ride is better than a 1st class walk. Lesson learned.
The questions are:
1.) how is the fuel consumption calculated? (perhaps a flow meter of some kind):thinking:
2.) How accurate is the distance to empty supposed to be? :thinking:
Keep in mind that this car is almost 11 years of age - e46 323i, 2000 model.
It would be interesting to compare.
Thanks.
F

 

[Coisman]

imp:
Frikkie, I've driven mine till it said I can still go 3km, thats right, 3, and then I stopped and filled up.
Also my consumption looks to be quite on the money, maybe yours need a good reset :thumb:

 

[Sankekur]

I found my average fuel consumption to be pretty accurate, but the distance I can still travel is usually a bit off (or at least I would guess so, never actually tried it out, for obvious reasons ).

I would assume that the consumption (instantaneous) is measured by a flow meter (l/t) and the current speed (km/t). (l/t)/(km/t) = (l*t)/(km*t) = l/km (something like that)
The average consumption would then just be the average of this.

The distance that can still be travelled I am not to sure about, it seems to use some stored base values (as it still gives values even after a reset) plus it also seems to use a calculated average consumption over a shorter time Edit: (and of course the amount of fuel left in the tank ).

 

[frikkieh]

imp:
Frikkie, I've driven mine till it said I can still go 3km, thats right, 3, and then I stopped and filled up.
Also my consumption looks to be quite on the money, maybe yours need a good reset :thumb:

Damn, your car is good

 

[Coisman]

imp:
If you drive it constant, like on the highway, it becomes EXTREAMLY accurate.
My experience was going to lichtenburg to visit my sister, and I made it into lichtenburg with the computer showing I can still go 3km,
I was sweating buckets full, but made it to the sasol garage in the main road in lichtenburg.
In town the lowest I've gone was about 38km left, then I put in feul.

 

[P1000]

There is definitely no flow meter in the equation. Your cars have modern fuel injection systems, in which fuel can be very accurately injected down to very small amounts. The computer simply calculates how much fuel it has injected and since it knows how far you have traveled, the rest of the equation is simple.

Now since the injection time can be off slightly, due to ageing injectors becoming sticky and taking a longer time to open and close properly, there can be some inaccuracies. Overall, it is supposed to be just an indication of consumption, and it is actually very accurate, given all the variables. Now distance to go is determined by a running average consumption over a window of time and distance, and also by the amount of fuel in the tank. This whole system is self calibrating (which requires you to fill to the brim every now and then to work properly), but sensors do fail and this is the weakest link in the chain. The sensor in question would be the one in the tank, which is very primitive and should last very well. I for one never let myself get close to such a situation, since it is very uncomfortable walking for fuel, as noted by the OP, but more importantly, you do not really want all the junk collected in the fuel tank to get pumped through your fuel system, clogging up everything. (Of course, since I drive a diesel, this is much more of an issue for me than for anyone with a petrol car.)

 

[Skwinty]

This whole system is self calibrating (which requires you to fill to the brim every now and then to work properly),

I agree with your sentiment here.

Whenever I put petrol in, I always fill the tank. The R50 buck top ups are pointless unless of course you are broke.

My MINI has almost 100k on the clock and I have always found the fuel consumption calcs to be accurate.

I have even put a 5l can of gas in the boot and driven until I ran out. It was spot on.

 

[Spy007]

had mine down to 1km left and made it. Remember it also depends on how you drive a it updates constantly. Perhaps reset and then have a look.

 

[328ii]

My experience is that tire sizes play a part here.

So the obvious question is what tires do you have fitted..?

Especially the profile

8?>

 

[frikkieh]

My experience is that tire sizes play a part here.

So the obvious question is what tires do you have fitted..?

Especially the profile

8?>

Front 225/45ZR17
Rear 245/40ZR17

I am thinking of replacing the fuel tank level sensor first.:thinker:

 

[Jandre]

My 540's range display gave me ---- the other day and I was still driving. The reserve light wasn't even on yet.

Fair enough I was nailing it. went from 70km left to ---- in about 300 metres.lol

 

[Hellas]

lol, on the 540 the 'km left' display is like a clock counting down while you drive

 

[Jandre]

Only problem is that it seems to be out of calibration quite a bit.

I drive like 5km, and loose 50km on the range indicator.

 

[Skwinty]

It works on the average consumption as well as the way you drive.
So, the computer needs to gather enough data to calculate an average that is meaningful.

Now according to the law of averages:

"If your head is in the freezer and your feet are in the oven, you should feel just fine"

Initially not so much, but in half an hour, yes.

 

[netercol]

"If your head is in the freezer and your feet are in the oven, you should feel just fine"

classic!

 

[Iceman007]

imp:
If you drive it constant, like on the highway, it becomes EXTREAMLY accurate.
My experience was going to lichtenburg to visit my sister, and I made it into lichtenburg with the computer showing I can still go 3km,
I was sweating buckets full, but made it to the sasol garage in the main road in lichtenburg.
In town the lowest I've gone was about 38km left, then I put in feul.

Yip mine is also spot on the lowest I went is 10 Km.

 

[Philip Foglar]

Mine is fairly close to being correct - I always fill up before 50 km remaining begins to show, basically when "Moaning Myrtle" starts to talk which is around 70 - 80 km range remaining. But if I fill the car and do the calculation myself the overall average consumption is just slightly optimistic of what the actual consumption is. I think this also depends on how many trips from cold start the fuel is consumed. I remember one morning still during the last months of Winter going into town for a few things knowing that I had to also fill the car - the car showed 60 km left when I pulled out of the driveway. Then as I was driving the range went down to 55 km and with less than a few km travelled it was on 50 km!! Then I presume as the engine warmed up the consumption improved and it was showing 54 km remaining - by the time I stopped at the petrol station it was bang on at 50 km!! LOL

 

[andrewbuch]

I find mine will show an average of like 10.9 but if I work it out according to fuel used & km travelled i get like 11.1.. so slightly out...

Have never gone below 50KM left as I usually try fill up when the orange light comes on (also only got the BC button OBC a few tanks ago)

 

[cuteboy]

On most cars you should try and keep your tank above 1/4 full.. You will get the correct consumption on tank to tank full fillups.. Travelling place to place on empty just ends up in your jerking and you kicking yourself.
Fuel lines and filters clog...and pumps start sucking air.
Nobody looks for leaking fuel lines anymore when they complain bad consumption

 

[DeanoBlado]

Bmw recommends not driving below 50km on the trip advisor