Hedley, there's been some good views posted on this and then one or two that are demeaning to you. Don't let the latter get to you.
A few times (ok many times) in the past I have had sups give me the same crap you outlined. The reason you are being hounded is because your sup is accountable for your "over allowed" to the center manager. He's just wanting to look better on paper. Next time he even mentions you being over, tell him to brown up, get in the truck and observe. Then if he rides with you and he even as much as holds a door open for you or turns a box label up, anything that will help you out,tell him to stop. Be sure you take your required meal time. Don't ask him to demonstrate or train/show you how it's supposed to be done, most anyone can work like wildfire for 15 minutes but can't keep that pace up all day. Then at the end of "the ride" calmly ask him to tell you where you did anything wrong. The AM harassment will stop......... for a minute.