It's only flat on the bottom

Tip for November 2009:
Flatten Those Tires

Scale tires often lack realism because they do not deform where the weight of the car presses the tread against the road surface. Instead, the car seems to "float" on perfectly round tires. To give your model a sense of weight, create a small flat spot on the bottom of each tire. You can make a larger flat spot to indicate a tire with low pressure. Just be sure to check your work and verify that all four wheels are touching the surface!