Hi, This is something which might have happened if your car went into a pit after filling the gas to fill and as the car is old, it might have
1. got stuck some where and you can bring it back to normal trying to go in the pit once again

- not recomended.
2. Ask your mechanic to look into this you go for oil change next time unless you want to go right now in case u runnin out of gas and u wont come to know.
3. in some old cars, the mechanism used to be quite manual, the way you have the water level indicators in the old water storage tanks and if the tank if full, it will not come down even if u switch off the car... (You can check the gauage again after driving 100 miles or so and u can be sure about the $3 in that case)