After you got the gas stamps, you will have to go from gas station to gas station to sell them. You have to sell a total of 6 before the time runs out. Make a good use of way-points on your map, they will help you get to them faster. Start with the gas station nearest you. Then there will be one to the North-East of your map also near you. After that, go to the gas station to the South-East, then to the center, and then the South-West side. Then your final gas station to go to is directly east of the one you were just at. You will go under a tunnel and end up at your final gas station. If all done correctly, you should have plenty of time to spare. Plan ahead and use of the way points, they will help you immensely.