It is not a "loose ground" or an "electrical short". It is likely a combination of conditions. Static as you know is a process of friction generating current. Type of clothing materials, environmental conditions (such as a dry windy day), type of shoes, movement of you feet across the cars carpet, your bodies natural resistance levels, etc. all contribute to this process. The shock you feel when you touch something is this generated current leaving your body when it finds a path to ground. The cars actual path to earth ground is by way of the tires, and based on the tires design and construction this ground relationship can be affected. For example, if the tire has a high degree of silicate it is likely to take longer for this static electricity to leave yourself and/or the vehicle. Unfortunately static is a natural condition and if your vehicle experiences this more than "other" vehicles, then it is a combination of the right conditions producing the event at a more common degree, and/or types of materials used in the vehicle, your clothes (nylons, synthetics, etc.) and/or your brand and model of tire (tire ground problems, are not normally common issues. Most good tire manufactures take this into consideration with the tires design and intended use).
And like Groucho advises about a ground strap during a fuel fill, I concur...but you can accomplish the same result, by just touching something metal that has a path to ground, like a fuel station pump before/during refueling.
