Is the Gen 2 Volt really a road car? I drive a lot of highway miles on two-lane roads. When you sit in a car for twelve hours, waiting behind trucks because you don't have the power to pass is unbearable. How does the Gen 2 Volt do in, say, 60-80 mph acceleration, engine only? Where does Mountain mode come into this? Will driving in Hold or Mountain charge the battery while driving?
Yes and no, I would not have any reservation about taking the Volt on a road trip but 12 hours in any small vehicle might get a bit tiring.
Mountain mode enables you to use the gas engine to put some charge (less in the Gen II than the Gen 1 Volt) into the battery (assuming the battery charge has been depleted) at least 20 - 30 minutes in advance and anticipation of making a prolonged mountain ascent. Mountain mode will not fully recharge the traction battery. Mountain mode is largely a relic of the Gen 1 Volt. The fact is you can confidently ascend most any mountain road in a Gen II Volt in Normal , Sport or Hold Mode without experiencing reduced power whereas having a reserve battery charge might be needed in a Gen 1 Volt to maintain speed on a long ascent.
Hold mode is as it sounds, it will maintain (more or less) whatever the current traction battery state of charge is when you engage Hold mode and use the gas engine until you return to Normal mode or end your trip. In Hold mode the Volt functions much like a conventional hybrid; using a combination of the gas engine and the traction battery to propel the Volt. If you slow down then regen will put energy back into the battery. If you stop then the gas engine may stop and use the battery to propel the Volt at low speed until the traffic starts flowing again. If you engage Hold mode on a descent then the battery level may increase somewhat due to regen but if you were on flat road or ascending a hill the Volt would maintain the battery at approximately where it started when you engaged Hold mode.
If you have a long ascent it makes sense to have the battery at no more than 50% of charge when you crest the top of the mountain pass so you can maximize energy recapture on the descent. You can recover maybe half of the battery charge on a long descent using L mode on the transmission, little or no need to use the brake pedal. If the battery is full when you crest the mountain then there is little to no additional battery capacity to store the descent energy and the Volt will just dump the excess energy.
While there is electric only mode (Normal), there is really no such thing in the Volt as gas engine only mode. If you dip into the throttle the Volt may decide to use motor generator A (MGA) to provide additional power to the drive wheels along with the power that is being provided by the the gas engine. You can read about the Voltec transmission here: http://gm-volt.com/2015/02/20/gen-2-volt-transmission-operating-modes-explained/
50 to 70 mph: 5.2 sec (Normal EV mode), 4.3 sec (Hold Mode, gas engine + EV assist)
You can read the Road & Track Comparison of the 2017 Volt and Prius Premier here: