basically, your throttle body isnt open as much with no load on the car than it would be with you driving around at 3k, and im guessing you tapped your boost gauge to the intake manifold. so the cyls are moving quicker...needing to suck in more air so thats whats creating more vacuum. its quite normal. in my laser after i high RPM run and then left in that gear i would see as much as 25-30 inches of vac.


try to visualize your throttle body. it may only need 5% throttle to spin 3k in neutral. now giving it 5% throttle in 3rd gear at 3kRPM the car wont stay at 3k, itll come down in the rpms. youd have to open the tb more..which creates less vacuum as there is more area for air to flow. hope this made some sense.