At what RPM does a car give the best mileage?

Excuse me, could you please elaborate on the relationship between a car's RPM and its fuel efficiency? Specifically, I'm curious to know at what RPM range does a car typically achieve the best mileage? Is there a specific RPM threshold that most vehicles tend to perform optimally in terms of fuel consumption? Additionally, are there any other factors that can significantly impact a car's mileage beyond its RPM, such as the vehicle's weight, engine size, or the type of driving conditions it's subjected to?