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VSS is short for vehicle speed sensor. The reason you want high vacuum is because more load on the engine means lower mileage and the more load that's on the engine the more your vacuum drops (wider throttle opening). Wide open throttle will mean very low engine vacuum probably 0 inches or very close. Cruising at 55 mph under low load you'll likely see 12-15 inches of vacuum. At idle you'll probably see about 20 inches of vacuum and on deceleration with engine braking probably 22-25 inches of vacuum. This probably isn't the scientific explanation, but anyway you get the picture.
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