In many cases, the output from sensor is just grounded when tooth is seen, so 5v pull up pulls the voltage to 5v when output is not grounded, leading to 5v square wave. Not familiar with GM stuff, if that's the case, but still.
That depends on if you are using terminal C or R. C is always safe, as the pullup voltage is from Speeduino. R may be safe, but you need to test the voltage it outputs at all reasonable supply voltages in order to ensure it remains below 5V (5.5V absolute max), or you can fry the processor input. Some have tested at closer to 6V.
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