>From: julian_davies@xxxxxxxxxxxxxx
>
>This is the *ONLY* accurate way to measure average fuel consumption. Trip
>computers don't. They have no reference to absolutes (total fuel volume
>springing instantly to mind) only rate of flow and time. They also DO NOT
>work by dividing the mileage travelled by the fuel consumed, or vice versa.
>They work by integrating a very dodgy instantaneous fuel flow measurement by
>time, and doing the same with speed. I guess some of the better ones do a
>calculation involving total mileage, but the earlier ones only integrated the
>instantaneous readings, which lead to hilarious numbers.
I had a third-party computer that was very accurate. You put magnets on the
drive shaft, which were picked up by a sensor. It clipped into the fuel
injector leads, and measured the pulse width of the injector, and thus, actual
fuel going to the engine.
Calibration consisted of driving a measured distance and giving it to the
machine, which compared it to its measured distance. Fuel calibration was
admittedly less accurate -- driving until empty, re-filling, and telling the
computer how much you put in. But I found I could hit it within two significant
digits.
I recall than on long downhills, it would go higher and higher, eventually
flashing "99999" which I assume means the fuel injection computer had
completely turn fuel to the engine off.
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