Adding a material isn’t enough. Without a material, collisions don’t work at all. With a material, most of the time, objects don’t fall through the mesh. But sometimes they do. It’s an intermittent bug. And it can be reproduced with 55wheels.blend.
I’m thinking that parhaps actuator motions are being processed without proper checking for collisions, so that actuators can force collision violations.
So when you’re driving, you can accidentally cause a motion that violates collision checking, after which gravity takes over and the object falls.
What I need are useful answers like “sphere vs mesh works reliably, but mesh vs mesh does not”. I know how hard it is to do reliable game physics (see my web site http://www.animats.com), but falling through the ground is a preventable problem.
The FAQ should be up-front about this if there’s a bug.