What happens when you take a truck designed to get 20 mpg on a 1,000-mile road trip loaded to the gills with people and luggage? Does it do as well mileage-wise as when it's only hauling around a single occupant? More on that later.
Like the Tahoe Hybrid we tested in the past, the Silverado Hybrid uses the Two-Mode Hybrid system developed jointly with Mercedes-Benz, Chrysler, and BMW. The powertrain consists of a 6.0L Vortec V-8 with cylinder deactivation just like the many 5.3L-versions we've driven, but it also uses a late-intake-valve-closing, Atkinson-cycle to reduce pumping losses from the engine. The result is an engine that makes less power-per-liter, but one that also uses less fuel than a smaller displacement engine of the same output, hence the jump to 6.0L from the typical 5.3L. The engine bolts up to a hybrid system that takes the place of a typical automatic transmission, with three planetary gearsets like a normal transmission, but with two 300V, 80hp electric motors and no torque converter. At low speeds, the larger of the two motors, located where a torque converter would normally be found, powers the truck alone while the gas engine is off. At higher speeds, the second electric motor near the output shaft works with the planetary gear sets to mimic a continuously-variable-transmission. Other significant additions are electric power steering and an electric A/C condenser so that both can operate while the engine is off. The 2WD versions are rated at 21/22mpg and the 4WD models, like the one we tested, are rated at 20/20.
In the week that we had our Silverado Hybrid we covered a good portion of California's latitude from Orange County in the south to Shasta County in the north, with pit stops in LA County and Monterey County thrown in for good measure. With a photo shoot scheduled in Monterey, I headed up solo, keeping close to the speed limit. At first I tried to maximize MPG by looking at the instantaneous MPG readout in the dash, but soon I discovered satellite radio. What can I say? I have a short attention span. With the cruise control set I was soon toggling back and forth between XM stations to stave off the boredom of driving up California's Highway 101 at night. After an overnight stay in Prunedale, California, and about 100 miles of mixed highway/urban driving, it was off to Monterey for the photo shoot and my first fill-up since leaving SoCal. After 442.7 miles the Silverado took on 22.077 gallons for an average of 20 mpg, right on target.
A good portion of the Hybrid's highway fuel economy had come from the 3.08 gears in the axles, which kept the 6.0L V-8 at around 1,500 rpm at freeway speeds. In addition, a soft tonneau cover served double-duty by lowering drag and concealing any gear in the bed. When we had the Silverado in town, the electric motor handled everything until speed hit about 30 mph if we were gentle on the throttle and had a level road to deal with, but with normal (for us) acceleration, the gas engine fired up at about 20 mph and shut off at around the same mark when we were braking to a stop. The regenerative braking worked virtually seamlessly with the hydraulic brakes-we only noticed the transition on one occasion. The only major difference in performance we noticed was at wide open throttle. There's a hint of lag before the gas engine kicks in, and then a boost of power as both engine and motor work to accelerate. The CVT-like aspect of the two-mode transmission kept the engine in the 4,000-5,000-rpm range while it shifted, there were no huge drops in rpm like in a normal automatic.