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"General Motors unveiled working versions of its large SUV hybrid at its Milford proving ground in Michigan, The Detroit News reported.   The and GMC Yukon will offer a gas-electric hybrid option \u2014 which GM officials claim will improve the vehicles&amp;#039; gas mileage by 25 percent \u2014by the end of the year, the story said.   &amp;quot;We&amp;#039;re still in the development stages, but we&amp;#039;re on track to have these vehicles ready by the fourth quarter of this year,&amp;quot; said Larry Nitz, GM&amp;#039;s executive director of hybrid technology.   The hybrid system powers the SUVs using electric motors mounted inside the transmission. It can operate using a gasoline engine alone or in combination with the electric motors. Additionally, the electric motors can be locked out of the drive train during times the engine&amp;#039;s power is needed, such as in towing up to 6,000 pounds, the News said, citing Tim Grewe, &amp;#039;s chief engineer for rear-wheel drive powertrain hybrids.   Both models include a valve shutoff system that allows the 6-liter V-8 to operate on half of its cylinders to help conserve fuel when at cruising speed.   A computer monitors the entire system and determines every 1/100th of a second what method is the most efficient means to propel the vehicle, Grewe added. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.roadandtrack.com/car-culture/a13073/gm-shows-hybrid-versions-of-its-chevrolet-tahoe-and-gmc-yukon-suvs/>