Demigan wrote: You could create tunnels with 100% grip (you can stand still on them while upside down) that go vertical.
Do you know how gravity works?
Don't you mean magnetised?
We have cars that flip their downforce on a whim without changing their outer facings, I think we can have a bit of suspense of belief in here, if it actually is suspense of belief...
Grip can be aquired in different ways (Like buying it on Steam now!), although most come down to the same principle in the end, and it becomes a bit contradictory. (Now I'm sure I'm going to make a few mistakes below but the general gist should be right):
A smooth surface in general seems like it will generate less grip, right? Since you slip more easily on them. However, a smooth surface also means more of the wheel surface touches the ground, allowing for more... grip. That's why race-cars have smooth wheels, it increases the surface area of the wheels on the track. Of course if there's water on the track it won't be able to get away causing much easier aquaplaning.
But there's more. Every surface that touches another surface, such as a car wheel touching the ground, has some molecules form bonds with the ground due to localized extreme pressure on those molecules, litterally making the wheel and surface part of one object. Of course, these bonds are easily broken because there's not many of them but this plays a large part in a surface's "stickyness". Race car wheels, similar to aircraft wheels, heat up from all the friction they endure, and melt slightly. This is done on purpose as the rubber starts to stick more to the ground, allowing the race car to do tighter turns with it's increased grip. It basically allows for the molecules to form these bonds more easily, add the large surface area and you are set for a whole new dimension of grip on your wheels.
And lastly, if one of the surfaces is rougher, say the ground, there will be more molecules that get the extreme pressures to bond with the ground, also causing more grip. This one seems the least useful to generate grip in general, as it seems to only work on tough surfaces and any surface based on loose material (sand for instance) would not work as well as simply increasing the amount of touching surfaces.
Now for Grip, my money would be on a special semi-sticky surface, like a giant sticky note that is easy enough to pull loose but somehow is sticky enough to stay on something. It would use the same principle as the semi-melted race-car wheels.
Magnetism doesn't have anything to do with grip anymore, but with attraction forces. While visually probably a better explanation for most people, I think we can safely use some kind of super-surface, even based on Van Der Whaal Forces (same stuff that allows insects and Ghecko's to walk on walls without using any adhesive material) to do it. In fact, we already are producing stuff that might allow humans to climb walls without problems, so I wouldn't be surprised if entire walls and ceilings couln't be made for vehicles to drive on.