What is the absolute greatest feasable yield in gasoline from one barrel of oil?
Cracking and other techniques could allow refineries to make gasoline out of almost all of it. Typically, about 30% of a barrel of light sweet crude can be extracted as straight run gasoline; cracking, polymerization, and alkylation can result in a gasoline yield representing 70% of the starting crude oil. The issue is complicated because exactly what constitutes “gasoline” varies from market to market, as does the definition of “feasible.” The demand for gasoline, compared to other petroleum products, isn’t high enough to push for refining techniques to convert that last scrap of tar.
Especially since “that last scrap of tar” is the asphalt the roads are made of. 
One interesting point I found out recently about the creation of gasoline, is that it requires huge amounts of electricity.
One argument commonly used against electric vehicles is that it’s just moving the pollution to the power stations.
But, with gasoline you’re still generating a lot of pollution at power stations to power refineries and also from burning the gasoline in an engine.
A barrel of oil contains 42 gallons. The following are the average products created in gallons through the refining of a barrel of crude oil (differs per refinery, type of crude, location, demand, etc.)
Gasoline: 19.5 gallons
Distillate Fuel Oil: 9.5 gallons
Kerosene-type Jet Fuel: 4.1 gallons
Residual Fuel Oil: 2.3 gallons
Liquefied Refinery Gasses: 1.9 gallons
Still Gas: 1.9 gallons
Coke: 1.8 gallons
Asphalt and Road Oil: 1.3 gallons
Petrochemical Feedstock: 1.2 gallons
Lubricants: 0.5 gallons
Kerosene: 0.2 gallons
Other: 0.3 gallons
TOTAL: 44.2 gallons
Therefore, there is a processing gain of about 2.2 gallons per barrel.