I used to deliver sushi in a middle to upper-middle-class town. Generally, tips were all over the place, there was no real correlation I could find after more than a year working 6 nights a week.
The only thing that stood out was that no matter if someone avoided tipping or someone tipped generously — it always averaged out to $5 a house.

I remember one customer was keen on making sure that she was not going to tip me. She looked me in the eyes and slammed her door as if I would try to barge in because she didn’t want to tip.