It's not the first GM product to wear a hybrid badge, but the 2008 Chevrolet Tahoe Hybrid is the first full-size SUV to feature a full hybrid system similar to the Toyota Prius. What's the difference? So-called "mild" hybrids merely use an electric motor to help out the gas engine when extra power is needed. More complex "full" hybrid vehicles can actually propel the vehicle on electric power alone. In the case of the Tahoe hybrid, a new "two-mode" hybrid system is the big breakthrough. Co-developed with DaimlerChrysler and BMW, the two-mode system uses an electric motor in conjunction with the Yukon's standard V8 engine and four-speed automatic transmission. According to General Motors, the Tahoe hybrid will deliver 25-percent better mileage than the standard model by switching between city and highway modes. During low-speed driving in the city, the system allows the Tahoe to inch along on electric power only. Come to a full stop and the system shuts down completely to conserve energy. In highway mode, the electric motor provides extra power for acceleration while an advanced engine computer works to optimize the gas engine for maximum efficiency. Active Fuel Management shuts down half the cylinders under light loads while smaller tweaks like revised cam phasing and late-intake valve closure provide further incremental fuel savings. Sales of the 2008 Chevrolet Tahoe Hybrid are expected to start by the end of 2007.