Are car fuel consumption computer accurate
Discussion
My Audi S8 over estimates fuel economy by between 2-9% depending on the type of driving done and distance travelled. It has NEVER underestimated lol
That's on a brimmed tank every fill up basis.
Odometer is required to meet certain accuracy requirement - speedo isn't allowed to underestimate speed under any circumstances, but can over-read by miles....
That's on a brimmed tank every fill up basis.
Odometer is required to meet certain accuracy requirement - speedo isn't allowed to underestimate speed under any circumstances, but can over-read by miles....
Engineer1 said:
Haven't people "read" the speed at the OBII port and seen it matches the sat nav while the speedo is showing 10% higher. The Odometer would be odd if it miss read by as much as distances between motorway junctions etc would soon show it up, print off a route plan then compare the route and your mileage.
Distances on signs are just as much out as everything else.otolith said:
It's conceivable that the algorithm being used by some cars to calculate the displayed "average" fuel consumption is doing something other than adding up fuel injected and dividing by miles travelled since last reset.
Edit - reciprocal, obviously.
From what I know, they don't measure the the fuel. I was told many simply calculated it from numbers in the engine mapping. Ie basically a lookup table of pre programmed fuel numbers.Edit - reciprocal, obviously.
Edited by otolith on Saturday 18th August 13:33
I have also heard they may back calculate from exhaust lambda sensors which basically give you how rich or lean you are running. But no direct measurement of the fuel going in. Even di engines won't do this as they'll just use period and pulse widths to change spray on time.
I don't believe I've ever checked.
My Yaris doesn't have a consumption feature (the MK1 diesels don't whereas the petrol does - strange) yet I usually do some basic fag packet maths to get a fairly consistant 56mpg. My Saab on the other hand does and the reading is usually between 32 and 34 mpg, I've just taken it as gospel but it probably isn't far off.
My Yaris doesn't have a consumption feature (the MK1 diesels don't whereas the petrol does - strange) yet I usually do some basic fag packet maths to get a fairly consistant 56mpg. My Saab on the other hand does and the reading is usually between 32 and 34 mpg, I've just taken it as gospel but it probably isn't far off.
swerni said:
Roo said:
PanzerCommander said:
I find this with the 'Stang too, but then if like other posters have said that speedo accuracy comes into it then that might be why, I find that when comparing to the GPS speed measurement the 'Stangs speedometer is saying 70 when the GPS is saying 69.
They do seem to be incredibly accurate. Must be an American thing as they all seem to be like it.SSBB said:
You are also relying on the fuel station pump flow totaliser giving an accurate volume of fuel delivered. These could be out by a few percent if recent news is to be believed (think I saw the link on here somewhere).
Very true. In a recent AA report several fuel stations had equipment tested after customers complaining of short measures. I dont remember the exact figures but the majority were actually dispensing slightly more fuel than registered on the pump displaykambites said:
Always struck me as odd that people are intensely distrustful of their ECU's MPG reading, let trust the odometer when doing manual calculations.
That's because it's vastly easier to get an accurate distance measurement than it is to get an accurate 'fuel used' measurement.The old Mk3 Mondeo TDCi was hopelessly inaccurate on the hire car I had, about 10mpg optimistic.
What I was getting at is that if you have continuous time/speed/fuel flow data, you might decide that mean miles per gallon is a bad measure to display. You might decide not to count time spent idling. You might decide to take mean or median instantaneous fuel consumption or to exclude unusual hard acceleration or... I don't know if anyone does that kind of thing with the displayed mpg, quite possibly not, but it's certainly possible. I would imagine the distance to empty calculations are a little smarter.
Scuffers said:
Nope, most just use a calculation based on the fuel injector pulse width (with corrections for dead time, fuel pressure, voltage etc).
That's how I imagined they all got their data.distance to empty calculators are the problematic ones....
what most seem to do is take the average MPG from the last X miles and use this against what it thinks is left in the tank.
the problems with this are:
1) the fuel gauge in the tank is still very crude (float/potentiometer in an odd-shaped tank)
2) the last X miles average MPG can be wildly out of step with what mileage is to come.
what most seem to do is take the average MPG from the last X miles and use this against what it thinks is left in the tank.
the problems with this are:
1) the fuel gauge in the tank is still very crude (float/potentiometer in an odd-shaped tank)
2) the last X miles average MPG can be wildly out of step with what mileage is to come.
Gassing Station | General Gassing | Top of Page | What's New | My Stuff