The whole issue of static shocks in cars is a tricky one. It's usually a combination of factors:

- Different brands and model of tire will (depending on the composition of the rubber) insulate the car from the ground by different amounts, possibly causing static buildup in the car's body as the air rushes over it. This can be seen sometimes when you buy a new set of tires for your car and suddenly notice that you're getting shocked when you weren't before, and vice-versa. They sell straps to attach to your car that dangle and rub the ground, supposedly to fix this problem. I tried those and it didn't work for me, so I can only conclude that my problem was caused by something else (or that the straps were substandard somehow).

- Different kinds of clothing, shoes and seat coverings, and their interaction thereof, can cause static buildup as you get in and out of the car. For instance, sometimes you'll notice the passenger being shocked when the driver doesn't get shocked, or vice versa.

- Different behaviors when getting out of the car, as was mentioned before; if your hand is already touching the car's metal when your feet hit the ground, you won't notice a shock because it will discharge through the soles of your shoes instead of your fingers.

- Different climate and weather. I noticed that I stopped getting shocked when I moved to Seattle, though the car, tires, and wardrobe hadn't changed.

In other words, I don't think there's a single discernable cause for every instance of car static. I think it can be caused by a bunch of different factors.