Once the engine reaches operating temperature, the engine shuts down as the vehicle comes to a stop, around 12 mph, and doesn't come back on until the electric motors accelerate the vehicle from 12 to 15 mph. At that point, both the electric motors and the engine power the Tahoe, with the engine taking over at highway speeds.
To get extra efficiency out of the 6.0L engine, it uses a late-closing intake valve to create an Atkinson cycle. Essentially, the dynamic compression ratio is lowered because cylinder pressure is bled off when the intake valve is open and the piston is traveling up. When the air/fuel ignites, it expands to a larger volume than it was compressed from, which is more efficient. Additional fuel-saving methods were also employed. The bump in displacement over the standard 5.3L Tahoe drivetrain allows the 6.0L to run in four-cylinder mode longer, and at higher vehicle speed than a similar 5.3L.
All of this trouble for a fullsize SUV might make you ask, "Does a fullsize hybrid SUV make sense?" Let's take a look at the fuel mileage from our long-term test Silverado for comparison. After plenty of time to get broken-in, it delivers 14.8 mpg. Granted, it's a 4x4 and the Tahoe is 2WD, but it's still a pretty fair comparison. The 2-Mode Tahoe we drove got 20.1 mpg and was still climbing up, gradually, when we had to turn it in. Undoubtedly it would continue to climb up at least a few tenths, especially if you had an eye on economy, which we didn't. That's still a 36-percent improvement. With current gas prices, that would save the average driver about $1,000 a year. GM hasn't given a hard cost difference between hybrid and normal Tahoe models, but when optioned the same, the option seems to cost somewhere in the neighborhood of $8,000. So, like most hybrids out there, it's going to take a while to recoup your investment through fuel savings. That's the downside. On the plus side, the 2-Mode system was so seamlessly integrated into the Tahoe that you'll hardly notice it, except at the gas pump.