The eternal OBD is wrong claim ....

[LTD]

We've all seen the claims/evidence that the OBD displays on our cars in incorrect and that our displayed MPG figures are wrong.

Wrong, you say ? Yes, wrong. Apparently .....

Now, that got me thinking. What makes it wrong ?

Is it wrong due to an incorrect measurement of the fuel used or an incorrect resolve of the miles travelled ?

I doubt our cars actually measure the amount of fuel used from the pump's flow-rate but back calculate it from a distance/speed perspective. Figures easily obtained from your cluster.

Now, I ponder.

If the OBD mpg is truly wrong and over-reads, does this mean that the actual mileage of our cars is higher than it truly should be.

In other words, are we being told that our cars are worth less at point of trade and are we being told our cars need servicing before they actually should ?

 

[SavMan]

I don't think it does anymore than calculate the remaining fuel in the tank with a set series of tables stored in one if the control units that estimate the consumption. I went into STAR developer mode and reprogrammed several control units so it thought my E55 was a 220 CDI. My average MPG on the display was showing >40mpg when it was actually closer to 18mpg.

 

[Ierton]

I thought it used the pulse width of the injectors for mpg?

 

[LTD]

I thought it used the pulse width of the injectors for mpg?

I have no idea how it does the calculation but it's not correct according to many people.

How accurate does this make our recorded mileage and also our service requirements and, ergo, warranty ?

 

[renault12ts]

The system for measuring the mileage and that for the MPG are different. As above the fuel is not metered per se...but relies on the position of the fuel gauge in the tank...which is not very accurate. But, since you know your car you can rely on the system to prevent you running out of fuel.

 

[BTB 500]

Odometer reading comes from counting wheel revolutions, so is accurate (assuming the rolling radius hasn't been changed by fitting a non-standard wheel/tyre combination).

As mentioned, I think the fuel used is derived from injector pulses.

 

[LTD]

Odometer reading comes from counting wheel revolutions, so is accurate

When does it know that the tyres are worn or changed ?

The fact I am trying to show is that it is inaccurate

 

[grober]

One potential source of inaccuracy might be a conversion of litres/kilometre to miles per gallon assuming that the original calculation software was written using the metric system. I can recall people comparing miles travelled by onboard mileometer and satellite navigation but more in connection with speed limits than distance travelled. There will in addition be minor differences in rolling radius from different wheel/tyre combinations/tyre inflation even. Just a few possibilities.

 

[LTD]

The system for measuring the mileage and that for the MPG are different. As above the fuel is not metered per se...but relies on the position of the fuel gauge in the tank...which is not very accurate. But, since you know your car you can rely on the system to prevent you running out of fuel.

Dealer - what's the mileage on your car ?

Me - 140k on the cluster but we all know that is not factually accurate. Don't we ? And whilst we are on it .... what about the rebuffed warranty claim on the basis of a mileage excess at the point of servicing ?

Dealer ......

 

[LTD]

One potential source of inaccuracy might be a conversion of litres/kilometre to miles per gallon assuming that the original calculation software was written using the metric system.

1 litres = 0.219969 Imperial Gallon

Not too difficult and enough decimal places to make the error almost irrelevant.

 

[del320]

We've all seen the claims/evidence that the OBD displays on our cars in incorrect and that our displayed MPG figures are wrong.

Wrong, you say ? Yes, wrong. Apparently .....

Now, that got me thinking. What makes it wrong ?

Is it wrong due to an incorrect measurement of the fuel used or an incorrect resolve of the miles travelled ?

I doubt our cars actually measure the amount of fuel used from the pump's flow-rate but back calculate it from a distance/speed perspective. Figures easily obtained from your cluster.

Now, I ponder.

If the OBD mpg is truly wrong and over-reads, does this mean that the actual mileage of our cars is higher than it truly should be.

In other words, are we being told that our cars are worth less at point of trade and are we being told our cars need servicing before they actually should ?

Paranoid? I'm not paranoid - I know they're out to get me...


What with brim to brim mpg calculations in my little book over 60k miles and regular journeys over known distances - e.g. Edinburgh to Melrose - I would never rely on any figures produced by those cunning Krauts!

OBD? Pah!

 

[SavMan]

Injector pulse width does make sense as one point of measurement it would still seem that calculation is based on a table of data

 

[BTB 500]

When does it know that the tyres are worn or changed ?

The fact I am trying to show is that it is inaccurate

Of course - everything is inaccurate to some degree.

The percentage difference in circumference between new and worn tyres is quite slight (obviously varies depending on the nominal diameter), and is probably not that important in the overall scheme of things!

 

[m1br]

Fuel metering by injectors is incredibly accurate, this is used to calculate fuel used.

Distance travelled will be calculated from data supplied by ABS sensors and prop/diff sensors .
(not familiar with Merc method here can anyone clarify?)

Generally the sensor derived vehicle speed is accurate from my experience of reading the OBD2 data versus a GPS. You can read this out easily.
(GPS with the old SIRFSTAR2 chipset I calculated long ago to be around +/- 0.5%)

What happens is the accurate sensor speed is normally modified UP in the BINNACLE chipset to over read on screen.
There is a legal requirement to not under report of course, and manufacturers err on the side of caution here.
Often the binnacle data will be over by up to +10% at any given point.

It would seem that fuel consumption computers universally under report fuel consumption.
My last three car computers were showing mpg about 8-10% more than the accurate long term calculation from filling/trip on my spreadsheet.
Indeed almost everyone says the mpg on the computer is not accurate, so I would deduce the modified speed data is employed.
This is odd, when the sensor data is more accurate, but I suspect its become common practice for marketing reasons.

So to add to the OPs thoughts if you do many repeat long distance journeys like I do, and calculate distance using COMAND/Tomtom/Google and then check the binnacle,
you normally do find you have travelled a small percentage more indicated miles than any of them quote. Try it
So the mileage is indeed higher than 'reality' on the cars binnacle, but its a tiny fraction.

Steve

 

[LTD]

So, if the car is capable of providing accuracy of fuel used and also the vehicle speed to, say 1%, why are car manufacturers allowed to display an inaccurate mpg figure nearly 10 times as inaccurate ?

Marketing ? False possibly ....

 

[Colin_b]

So, if the car is capable of providing accuracy of fuel used and also the vehicle speed to, say 1%, why are car manufacturers allowed to display an inaccurate mpg figure nearly 10 times as inaccurate ?

Marketing ? False possibly ....

I used to own a BMW530. The OBC had a 'correction factor' option that was used to compensate for tyre sizes and such like. Thus you could get the OBD accurate. Pity MB doesn't have the same thing.

 

[Dieselman]

So, if the car is capable of providing accuracy of fuel used and also the vehicle speed to, say 1%, why are car manufacturers allowed to display an inaccurate mpg figure nearly 10 times as inaccurate ?

Marketing ? False possibly ....

10 times the reading? They are normally about 10% out.

The OBD calculates from injector pulse width and a calculated torque output per volume of fuel.
What happens is that the engine doesn't make the calculated torque output per cc of fuel, so more fuel has to be used to make the required output.

 

[Ndaa75]

Now im really thrown. M

I have a W209 200 Kompressor CLK, 163 bhp. According to MB figures, i should achieve approx 44 mpg on extra urban. Reality is i get more like 34. How can ths be so incorrect?

 

[gIzzE]

Now im really thrown. M

I have a W209 200 Kompressor CLK, 163 bhp. According to MB figures, i should achieve approx 44 mpg on extra urban. Reality is i get more like 34. How can ths be so incorrect?

That's a completely different matter, my guess would be that you don't understand what 'extra urban' is?

 

[gIzzE]

I asked this question when I put winter tyres on, my speedo then matched my gps speed readings, my mpg also went up from 34 to 37, and I was expecting it to drop with the winters on.

I was assured that the odometer was calculated differently to the speedo and the mpg readout.
I am not so sure that they are all so different.