The new two-mode hybrid system uses a transmission combining both mechanical gears and electric motors. In GM's heavy SUVs (5,500-plus pounds) I've found it returns mileage in the low-to-mid 20s around town, just as GM claims. The same Zeta platform (that's GMspeak for the underpinnings for a bunch of vehicles) is seen also in offerings as disparate as the Pontiac G8 and the upcoming Chevrolet Camaro. GM trumpets the Denali as being the first vehicle in the GM stable to have both an E85 engine (that burns an 85% ethanol, 15% gasoline mixture) and the hybrid drivetrain, although E85 is pretty well known technology in GM circles.