C. E. White wrote:
>
> "Steve" <no@[EMAIL PROTECTED]
> wrote in message
> news:QtidnVHhHpXvdLnVnZ2dnUVZ_sPinZ2d@[EMAIL PROTECTED]
>> Bailey B wrote:
>>> Some new cars have digital mpg sensors
>>
>> Its not a sensor. Its the engine management computer that does the
>> math to come up with mileage.
>>
>> and gauges. For those of you
>>> that have them, how accurate do you think they are?
>>>
>>> When on interstate how much difference is their in mpg at 55 mph, 65
>>> mph, and 65 mph?
>>>
>>> A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg
>>> on stop and go gravel back roads.
>>>
>>
>> The one on my wife's 1993 Chrysler 3.5L is EXTREMELY accurate. And it
>> should be, after all the computer knows exactly how much fuel its
>> putting through the injectors on every pulse, how far the car has
>> gone, and how many pulses of what duration were required to get there.
>
> Well actually, the PCM knows how long the injectors were commanded to be
> open. If you know this, the fuel pressure, the injector flow rate, and
> the miles traveled, you can estimate the mileage.
Yes, and all those ARE precisely known unless there's a system fault...
and then you'd get an error code. Even though fuel pressure may not be
measured directly by all EFI systems, it is correlated to manifold
vacuum in a fixed, precise way, and manifold vacuum is measured.
However, over time
> both the fuel pressure and the injector flow rate can change and changes
> in tires can definitely affect the miles travel component of the
> estimation.
Not much. Changing tire SIZE can have a big effect. Inflation- not so
much.
I assume the manufacturers build in some compensation for
> changes, but I expect over time the estimate will vary. Cars I've owned
> with mileage meters generally overestimate the mileage slightly, but
> they still are pretty good. My current Ford Fusion overestimates the
> mileage by around 0.5 mpg if you compare the mileage calculated from gas
> purchases to the gauge estimate.
I'd trust the built-in system more than I'd trust my ability to re-fill
the tank to EXACTLY the same level on two given fill-ups. I'd want to
average over 5-10 tank fulls before I'd claim that my measurement was
more accurate than the built-in system.
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