it might be something wrong with the tank floater mechanism... a lot of the times in older/unused cars it fails. A foolproof way to know is to go to a gas station and fill your tank, then pull into a parking spot at the gas station/a neighboring parking lot and let your engine idle for a moment or two. See where the needle goes to. If it doesn't go to full, pull your gauge cluster, pull the needle, and physically move it to full... a lot of the times if anyone has messed with the gauge needles at all, the needle for the gas level got knocked around. Chances are in the favor of it being a floater thingie. But I also go by mileage, so it's all good.. If you know for sure you get 239 miles to a tank, then you have no chance of getting stranded, unless you forget to check your trip and try to go for 240 miles.
