I was transporting some equipment for a client several hundred miles. They insisted on following my truck and were very adamant that I go the exact speed limit. The following took place at the first rest stop.
Client: You were speeding. We had to catch up to you several times.
Me: No. I made sure to set the cruise control at exactly the speed limit.
Client: Yeah. But your engine is bigger than ours.
Me: … Haha … huh? … Ha… Wait. Are you serious?
Try as I might, I could not impress on them that 60 mph is the same no matter how big the engine.