My idle is pretty smooth. Now that I thought about it I think the gauge actually reads 20" at idle. The needle points at about 9 O'clock position which is directly opposite from 15 psi on my gauge so I just assume it's 15". I just looked up a pic of the Autometer gauge that I have and it actually says 20 at the 9 O'clock position... So I guess I'm at 20" at idle and ~25" coasting. I will check when I get home. Thanks.