Indeed, the reduced displacement, Atkinson cycle, CVT ratios, LRR tires, and improved aerodynamics of some hybrids improve highway fuel economy. GM's dual-mode system adds to that a planetary set of four fixed gears that provides a direct mechanical path for the electricity.

The Tahoe's additional 2 MPG on the highway is rare among heavier hybrids. With HSD on the LS and GS, highway fuel economy drops by 2 MPG.