Yeah, trucks aren't usually referred to as cars, unless we don't know if it's a truck. I mean you always hear "what car does he drive?" and the answer can be anything from a typical four door, to a truck. Or "how many cars does he own?"- who says there can't be a truck in that list?
I believe a truck is a division (not sure if that's the word) of a car, but the word car can still be, and is usually applied to a simple passenger vehicle.
Kind of like:
-Vehicle
--car
---truck
---passenger car or just a car
---(other similar ones)
--bicycle
--train
And so on.
Both a car and a truck typically have four wheels, can transport passengers and carry it's own motor, but they can't both be called trucks, while if you call them both 'cars' it would be acceptable.