What is the best rev range for fuel economy?

I'm curious, what would you consider to be the optimal rev range for maximizing fuel economy in a vehicle? Are there certain RPM levels that tend to yield better mileage compared to others, and if so, what are those ranges? I'm interested in understanding the science behind it and how it impacts overall performance and efficiency. Additionally, do different types of vehicles, such as gasoline-powered cars versus electric or hybrid models, have different optimal rev ranges for fuel economy?