Going down hard surfaces like asphalt, hard packed dirt etc. mine usually shows about 8 inches of vacuum. In deep sand at a stead state it shows about 2-3 psi.
I did notice at Taylor Park that it depended on the direction I was going. South there was a slight uphill back to camp and at 35-40 it would boost at about 2-3 psi. On the way to the trading post which was ever so slightly downhill, it would run 0 or lower into the vacuum.