How does the Overhead MPG Computer work?

I was just curious if the readings the computer take are from a flow rate meter or some other method of determining mpg? When I use it on the Hwy, I
get a pretty accurate reading when compared to how much fuel vs miles on the odometer. However, when I use it around town It will read say 15.2 mpg and my fuel calculations will be 1 to 1.5 mpg higher. Thanks in advance
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hawkdoc wrote:

I don't know about the new ones but older ones use a vacuum sensor reading and converted it to mpg.
JAM
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The computer uses an algorithm that factors in fuel pressure and injector pulse width to calculate fuel flow.
Hope this helps!
Mike

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Based on various engine parameters, the computer knows how much fuel it's squirting (or should be) via the injectors. It takes that, combined with the odometer information, and calculates an average. So, if you put in higher-flow injectors, it throws the calculations off, because the computer is assuming a flow of 'x' amount of fuel, not '1.2x'. The same is true if you have clogged injectors.

If you're using the average (some years also have an instantaneous display), that's an average over (I think) the last 30 miles, so it could take some time to adjust to the lower economy of city driving.
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