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The mpg bouncing up and down on the instant mpg is normal. If the speed is bouncing up and down at a steady state speed then it is not reading correctly and needs debounced on a Metro. A lot of other cars won't be able to be debounced because they are not mechanical switches generating the VSS signal.
Something like my Astro has an electronic module generating the VSS and there is nothing that can be done to clean up the signal from it if it was noisy. As far as I can tell the Metro is about the only car that uses a mechanical switch to generate the VSS signal.
So to do it simply get a resistor, it doesn't have to be exactly 1k it can be 1k to 10k and still work so if you have one laying around use it. Connect one side of the resistor to the VSS wire going to the mpguino and the other side to the ground wire going to the mpguino. That will most likely be all that is needed to get the signal clean. If it is still bouncing around some then you can add a second resistor one side connected to a switched 12V source like the radio or heater blower fuse the other to the VSS wire going to the mpguino. You can't use the power wire going to the mpguino since it is 'always on' 12V.
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