"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. 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. 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. Of course I am never sure
exactly what the gauge estimate reflects - is it the average mpg since
it was reset, or the mpg for the last X number of miles, or ? And my
"calculated mileage is actually a little lower than actual since my
odometer reads about 3% slow (I've actually gone 100 miles when it
says I've gone 97).
Ed
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