A small penis truck,is usually but not limited to having a huge lift kit, bright or exotic colors, chrome rims, and is always clean. Not to be mistaken with a off roading jeep or the like. It is always driven by a man who has to compensate for his dick size, by driving a truck that is bigger than what most men have.
Kid: Why does Bob our next door neighbor have a pick up truck that has a 5 foot lift kit, a bright yellow paint job, and chromed rims?
Kid's mom: Becuase his dick is the size of a shrimp and he has to compensate by driving a SPT or small penis truck.
Kid: How do you know he has a small dick?