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ABS sensors use a gear like wheel with a magnetic pickup. Similar thing is used for OBD 2 cylinder balance counting 105 flywheel gear teeth per engine revolution on a 1995 Nissan Sentra, for passive cylinder balance calculations while engine is running without disabling any spark plugs.
Old tech cruise controls used a simple magnet on the prop shaft that gave a single impulse signal per revolution of the shaft. Differential ratio would tell you wheel speed relative to prop shaft speed.
Not necessary on front wheel drive, since half shaft speed is same as wheel speed.
Don't think a reed switch would hold up long.
regards
Mech
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