I recorded 10 mpg while towing, about the same I've experienced towing similar sized boats with large conventional-powertrain pickup trucks. GM's Vehicle Line Director for full-size pickups Mike Tulumello acknowledges that two-mode offers minimal efficiency benefit on the highway and practically no mileage benefit while towing over standard powertrains.
The situation changes for the better in urban driving. In that circumstance, braking regeneration and electric propulsion combine to significantly curb fuel consumption. During a 30-mile tour of practically every San Antonio neighborhood, I achieved an amazing 25 mpg. Without causing any significant hindrance to traffic, the Sierra's electric motors handled propulsion duties most of the time with the engine starting only occasionally to power one of the electric motors operating in generator mode. After long runs using the electric motors, the battery is depleted so the engine must run to restore its state of charge.
As long as regular gasoline continues selling for less than $2 per gallon, two-mode - and for that matter, every other hybrid--is a tough sell, despite the $2200 energy credit that can offset federal tax liabilities currently offered on the 2009 GMC Sierra Hybrid. But this leap-ahead technology will soon enjoy its day. When gas prices again surge past $3 per gallon on their way to $4, investing a few thousand dollars to gain five or more mpg will seem like a genuinely shrewd move.