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Oh, and after looking on that forum that evo-move suggested, I'm now fairly sure that there is a speedometer cable running from the transmission to the instrument cluster, and that there is a device in the instrument cluster that turns the speedometer cable movement into an electrical signal, that is then sent to the ECU and whatever other devices need it. Which makes me worry once again about the speedometer sticking, and affecting the MPGuino data. (fortunately when I've seen it happen it doesn't last for too much of an extended period, so hopefully whatever impact there is, is limited)
Two interesting things I noticed with the MPGuino. One, the speed can jump around by as much as 2 mph (though usually less). I'm guessing that, on average, it turns out as a good approximation of the actual speed? And two, when coasting at higher speeds while still in gear, the fuel use drops to 0. I'm a bit surprised, but hoping it is not just lack of signal sensitivity.
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