Why can't hybrid cars get above about 40mph only using electricity? Is it because there is not enough power or because scientists don't know how to convert the power at higher voltage?

I just saw this post, so sorry for being so late with a response. I believe the answer has to do with a couple of factors. First, hybrid cars manufacturers need to balance the benefits of having the batery pack against the weight and space penalty. Hybrid cars are engineered the to use battery power for low-speed stop and go driving, and the gas engine for higher speed sustained driving. Consequently, the electric motor and transmission are optimized for low speed operation, and not for sustained high speed driving. Also the battery pack is minimal - it's not sized to provide enough continuous energy for high speed operation. This saves weight and space. But there is nothing inherent in electric vehicles that preclude high speeds. The Tesla and Chevy Volt are designed from the ground up for all-electric operation, and so are designed to accelerate and cruise at highway speed for sustained distances on elecricity alone. They have much larger and heavier battery packs and elecric motors than hybrids. The Volt will have a small gasoline engine, but not for powering the wheels - it will run a generator to recharge the batteries on the go if needed.