 While in this series mode at higher speeds and loads , ( typically above 30 miles per hour ( 48 km / h ) at light to moderate loads ) the gasoline engine can engage mechanically to the output from the transmission and assist both electric motors in driving the wheels , in which case the Volt operates as a power @-@ split or series @-@ parallel hybrid . After its all @-@ electric range has been depleted , at speeds between 30 to 70 miles per hour ( 48 to 113 km / h ) , the Volt is programmed to select the most efficient drive mode , which improves performance and boosts high @-@ speed efficiency by 10 to 15 percent . 
