Yes, all vehicle tires have treads on them. Truck tires will usually have larger and deeper treads than regular sedans.
Yes, the tread is much deeper as well as the rubber is much softer.
No, tires have treads.
The answer is: have
You will get the best gas mileage if you get the tires that fit your car that have the least rolling resistance. Rolling resistance is the amount of force needed to keep an object rolling at a constant speed. Heavier tires and those with deeper treads are less efficient. Remember to properly inflate your tires too.
Tires are pro-rated based on the amount of wear they have. This is largely determined by the depth of the treads.
Because the rubber in tires was being used for war supplies including tank treads and jeep tires.
Because the tire is very thin when there are no treads unless they are racing tires and it is very dangerous
Here is a good article that covers a lot of issues with tires:http://www.ehow.com/how_5077858_check-tires-maintain.html
You will lose traction especially on wet roads.
If the treads lack definition and/or if the car "pulls" to the left or right.
They are 255 mm wide over the treads.