From what I understand the Volt cannot do a maximum speed over 65 mph unless it is using batteries. So once the batteries are toast, knock it down to 45 or 50.
Second farce with the Volt, and long term owners will find out is that the batteries should be viewed as consumables. In three years if they perform to 15% of their stated "new" capabilties you might be doing good.
Someone mentioned the other day the gas engine will not charge the batteries. Ooops. Taht makes it unsafe on the freeway.