If many people switched to bio-diesel, how many cars could be powered only on used cooking oil?

Energy density says with diesel at 35.8 megajoules per liter and gasoline at 34.2, you should go a bit further with diesel.
Energy density Wikipedia
Diesel, esp. from petro sources, tends to create more hydrocarbon pollutants and particulates per distance traveled, esp on short hops where the engine doesn't warm up to it's higher efficiency range. Diesels like running on long trips.

Diesel engines tend to be noisier. Diesel could have issues with fuel gelling in very cold weather if it has insufficient winter additives, and sometimes even that isn't enough for a frigid weather startup..

Gasoline is more refined than diesel and likely has a bigger production environmental footprint than diesel, although diesel has a bigger combustion environmental footprint. Biodiesel can be better depending on its source.

Usually you do not have the choice of diesel vs. gasoline for the same model of car, so there will be other factors to consider besides fuel type. Best wishes shopping for your next vehicle.

Steam or electric are probably cheapest TO RUN... But most expensive to buy.... Butane would probably be next... It is actually a Mixture of Butane, Methane and a few others... In Australia it is the same as we use for the BBQ..... This would probably be next.... but unless the car is Factory converted,... then you will probably need some Petrol/Gasoline just to start .. You use maybe 2 or 3 Cupfuls each time so you don't use Much... It is also a Good backup if you run out of LPG....... The rest... it depends on th e Government and Fuel Companies which one is cheapest but what it SHOULD be id From cheapest up... Diesel, Unleaded, Premium Unleaded
Forget the rest... They would be cheap to run but the conversion would be so expensive it would be not worth the effort

The sun and the wind will both be our major source of energy in the future. The sun is always there and the turbines will be needed to supply energy to homes in the winter time. we will see only electric cars on the roads and the airline industry will have to figure out something cause planes do not fly on solar power.

If you hook it up correctly and the Voltages match... yes.

As for how long... THAT depends on a number of Factors... How much sun, What quality is the Solar Panel, Placement of the Panel.. the goes on and on... But you CAN charge a Battery...I charged a Battery for quite some time using a Large Solar array.... We were Camping an using the Battery for Lights Radio and Fridges.... So it CAN be done

Pretty cheap. You'll want to determine fuel efficiencies between selected car models.
Also consider how your fuel expenditures would be affected if you had a gas-electric hybrid?

In my opinion, the use of fossil fuel sources of energy will decrease in the near future. Meanwhile, other forms of clean energy will be cheaper and more available, like: solar, wind and hydroelectric energy.