The problem with that is that you aren't pulling vacuum into the cylinders you are pulling air through the intake at 17" of vacuum. That air is being compressed and burned at 10:1 not vacuum. Not only have you increased the drag by droping a gear you are still fighting the fact the the cylinders are still firing even though it's a lot less fuel and air. Your example would only work if there was a perfect vacuum in the intake (no fuel or air)which is not the case. Want to give the numbers on 8 cylinders compressing the same volume at 125psi in one revolution of the crank?. I think you'll find it's a lot higher. 17" vacuum = 5.8 psi.