Feedstock is the problem here: there's simply not enough net primary productivity on the planet, and humans already appropriate ~20%.
There are some marginal gains possible, but on the order of single-digit percentages of present fossil fuel consumption. Maybe low double-digits.
The US had a largely biofuel based transport system in 1900. Horses consumed 20% of all US grain production, the population was under 100m, and transport generally was a small fraction of today's values (or at least last year's) per capita: closer to 300 mi/yr, much of that walking, than 15,000.
There are some marginal gains possible, but on the order of single-digit percentages of present fossil fuel consumption. Maybe low double-digits.
The US had a largely biofuel based transport system in 1900. Horses consumed 20% of all US grain production, the population was under 100m, and transport generally was a small fraction of today's values (or at least last year's) per capita: closer to 300 mi/yr, much of that walking, than 15,000.