Vehicles have not gotten bigger and heavier, that is a common misconception. The >10mpg 70 Blazer I had could easily crush the biggest SUV's today.
Oil Companies are falling the same fate as most big companies where short term profit is most important to keep the stockholders happy, new product research and development takes a long time therefore the lowest priority. Companies like HP close their R&D and just resell other products, Microsoft's product has just gotten worse since 2000, Honda, Toyota, GM, etc just keep churning out the same old product with slightly different skin. Services like my cable internet are no faster today than 1998 when I first got it. Its going to take some garage tinkerer to come up with something new
Also its going to take a while to get crop production up to be able to support the extra needed to make it as well as slowly working out the gov subsidizing.