I recently made a mod that adds the ability to refuel a car into the game. 100% tank is enough for about 8 hours (game). With this mod, a nice gas tank is added in the lower right corner of the screen, showing how much gas is left.
In order to refuel, you need to drive up to the gas station and press Num 5.
A menu will appear with the cost of refueling:
20% tank — $100
50% tank — $300
75% tank — $400
100% tank — $500
To select, press the corresponding buttons on the keyboard (num 1,2,3,4).