Decade or two to ramp up production for new vehicle sales. Plus a decade or so for consumer acceptance lag. Then two decades or more to phase out existing gas cars. We're talking half a century here.
Yes, at one point there were 5 cars per million people period. Around the year 1890. Today there's 0.15 cars per capita globally. It took over a century to scale up that much, so I don't think that's the sort of point you want to be making. Plus, not only do we have to scale up for existing car replacement, but also to handle the rapid growth of the third world, which will push that 0.15 cars per capita way up over the coming decades. It's simply something that will take decades to get the production capacity in place, and then decades at that level to phase out the existing vehicles on the road.
Biofuels are hardly a gap filler. Have you ever checked how much land they eat up even to meet today's tiny pathetic percentage of the market share? To meet the needs of the average American driver's 12k miles per year in an average 24mpg car (500 gallons) would require 3 million square miles of farmland dedicated to it, more than double the US total farmland for *all* crops - for human consumption, for animal feed, for clothing, for industry, everything combined. And that's just for passenger cars, let alone freight trucks, trains, airplanes, ships, etc.
It's not a gap filler. It's an environmental disaster on a greater scale than the oil it's trying to replace.