An ICE is typically most fuel efficient at about 2000 RPM and 90% throttle (this is different for every engine of course, but those numbers are close enough for discussion). A typical car can be at 45% fuel efficiency if you can pull that off, but 90% throttle when cruising will bring your RPMs and thus ground speed up. A hybrid can use a smaller engine that can just barely keep your car moving at 90% throttle and use the electric to get acceleration up to those speeds.