Electricity has been the military’s fuel of choice for the very large (2,500-ton submarines) and the very small (12-pound unmanned aerial vehicles), but not for the vast middle ground of tactical vehicles. A vehicle maker hopes to change that with the HEMTT-A3, a hulking hybrid truck. Oshkosh Corp. says the vehicle consumes 20 percent less fuel, requires less maintenance than current trucks and can double as a power source for field hospitals or command centers.Dubbed the “heavy expanded mobility tactical truck,” the vehicle is headed to Army proving grounds this fall to undergo durability, reliability and performance tests.The HEMTT-A3’s power train provides for more flexible vehicle configurations. That has freed space for a 40 percent larger cab and allowed engineers to cut 3,000 pounds of weight off the 38,000-pound curb-weight vehicle.Fuel savings increased to 33 percent when the vehicle was driven with lots of stops and starts.