Why do some cars follow you?

Late '90s I would drive cross country to visit relatives every summer. On one trip something went out on my pickup and I couldn't do more that 50 on the highway. Can't remember exactly what the problem was, fuel pump or filter. I was young and didn't know how to fix it at the time

By tucking in behind a semi I could match their speed. As Farmerbob1 says: you do indeed have to be very close to get a draft. I had a CB and could talk to the guys I was doing that to. Most were ok with it, if they complained I'd let them move on. A few of the guys slowed down when they dropped me. I think they appreciated someone to talk to