The Tracks of My Tires

The police found a murder victim and they noticed a pair of tire tracks leading to and from the body. They followed the tracks to a nearby farmhouse where two men and a woman were sitting on the porch.

There was no car at the farmhouse and none of the three could drive. The police arrested the woman. Why?

Clue: The police didn’t ask any questions but merely used their powers of observation. When the police arrived, none of the three suspects was carrying a weapon or wearing blood-stained clothing. The police correctly deduced that the woman was the murderer.