I was told that parking on the grass or dirt will cause the body of a car to rust prematurely, as apposed to parking on asphalt or concrete. It seem that there may be some truth to it because the ground holds moisture but is it that much of a big deal. Maybe the Mythbusters can tell us.

Grass and weeds have a way of really rusting out a car. As far as dirt/soil it all depends on the moisture content of the soil. Which usually depends on the elavation as compared to the ground water. You could park a car on a sand hill and be fine. Park it on a sandy beach and have the car rust. It's even like that with parking on cement. A low lying cement slab will rust a car. A 'high and dry' cement slab would be much better. You could lay down a sheet of heavy plastic sheeting to make a vapor barrier under your car.

This holds true indoors as well. If your garage floor is at or very near grade, a moisture barrier between the concrete and your car will reduce the amount of condensation and subsequent corrosion on the underside.

Free air flow helps as well, so if the car is covered it's best to leave the sides and ends pulled up a bit so that air can flow. Cold air retains very little moisture, it's the temperature swings that bring on the water.

If you find a car that has spent a few years sitting in a grassy lot, the belly will be in deteriorating condition. Rocker panels and floor pans really take a beating from vegetation.