Here's a re-wording of WW's post:
If the pylon was fixed, it was there even if nothing was on it (P51, P47, P38, etc etc etc), and this drag is already in the FM. If the pylon was separate (109 rack, 190 rack, rocket tubes, etc) then after you use the item ON the pylon, the pylon is coded to add extra drag. Depending on the setup it could be as little as -2mph, or as much as -20 to -35mph (in a few cases).