size and quantity of the gas tank
How many gallons EXACTLY is a quarter of a tank of
gas in a 1993 Pontiac Grand Prix?
Look up fuel capacity in your owner's manual. I believe total capacity is APPROXIMATELY 12 US gallons which would make a quarter tank APPROXIMATELY 3.
Don't depend on a needle gauge. They are reasonably accurate but remember that manufacturers have the gauge point at E when, in fact, there is still about a gallon left in the tank. Since most modern cars rely on the fuel to also cool the fuel pump, it is never a good idea to run dry and the needle gauge will never be precise.