It is more economically viable then corn based ethanol, but still it can't and won't replace petroleum, perhaps dilute it to some extent. You just can't produce enough of it at economical prices to match global demand for motor fuel.
I think natgas is a much more suitable replacement for motor fuel. It can be used as compressed natgas to replace both diesel fuel and gasoline, and it can also be converted to methanol quite cheaply to be used as a liquid fuel replacement (or dilution). Either way to use it doesn't require too much adaptation of infrastructure and engines.
In order to use crops based fuel efficiently there must be a technological breakthrough, one that will allow efficient converting of cellulose to fuel, or an efficient way to grow algae which yields vast quantities of oil.