Vice City Car Carrier?
 

If you go to the docks, there is a car carrier which you can drive around. I thought that you would be able to carry cars on it, but when you drive away, the cars just roll down (as if they are not moving relative to the ground). Is there anyway to use the car carrier, or is it just a tease?

It makes a nice moving ramp.

Kellehair said it. It really just functions as mobile ramp.

Strangely enough, though my first reaction was to use it as a jump, I've been thinking about trying to carry a car around on it.

Maybe you can lay a motorcycle down behind a car? I'm away from my PS2 so I can't try it right now.

Last night I was going to push a car sideways up the ramp with another car, but the damn ramp truck disappeared while I was jacking the second car.

That's what happened to me too.

Finally pushed a car up onto the carrier sideways. Even tried laying a bike down behind it.

Guess what, you still can't carry it around. Apparently GTA's physics engine doesn't account for friction. The objects are "parked" and there's no outside force working to move them in a horizontal motion. Thus, they simply slide up and off the ramp, no matter what you do.