In general that's true, but not necessarily for a hybrid. Toyota's HSD does not use anything remotely resembling a regular transmission. Instead it uses the ICE, motor/generators and a power split device (PSD) to perform the same function. For the most part while cruising along running on ICE (above 42 mph) the engine will run at a constant RPM regardless of speed. That means it's using a constant amount of fuel per second so the higher the speed the more distance you're covering for that amount of fuel. It's not until the speed gets higher that the aerodynamic drag starts becoming a major factor (guessing at 65 mph) and fuel efficiency starts decreasing as speed increases.