you can fill a tank of gas from 7/8th full. I've made my tank of gas last months in the past. recently half tank last me like 3 weeks.
yes getting an electric car will help you dodge gas prices, but not necessarily oil prices, because your connected to the electric grid and most power comes from natural gas which comes from oil wells and oil processing.
now that I answered those question, the logic is dead simple every gallon of gas you buy today will be cheaper than the gas you buy tomorrow, if you want to save as much money as possible you buy as much gas as you can put in your car as soon as possible.
the logic would conclude that you buy as much gas as you would use for 3 month or however long you think oil crisis will go on