When your tires have 2/32" tread depth left you need to replace them.
we need rough tyres to increase friction
If you're having to put air in your tires frequently then your tires probably need replacing. Also if the tread on the tires is worn down they should be replaced (a symptom of worn tread is the tires squealing on the road for no apparent reason).
More than narrow tread tires.
Tires have molded indicators called "wear bars." If these are visible across the tread pattern, your tires need to be replaced.
There are tread wear indicators that are in the tire itself. When the tires need to be repalced theese indicators show.
I inspect the tire and measure the tread depth.
Minimum tread depth for all 4 tires is 2/32"
If you want traction you need the tread. Smooth tires are only used for racing.
A good test is to take a penny and place it in the tread grove with the head of Lincoln facing down. If you can see the top of Lincoln's head you need new tires. If the tread is worn more on the inside or outside you need a front end alignment. If the tread is worn on the outer edges you are running the tires with too low air pressure. Worn in the center, too much air pressure. Of course dry rotted and cracked tires need replacing no matter the tread depth.
In the tread there are small 'bars' at the bottom surface about 2-3 mm high, when average tread wear is down to these 'bars' you should get new tires.
You put a penny into the tread with Abraham Lincoln's head facing the tire. If the tread covers up Abe at all, then you're good. If not, get new tires!
It does not! In fact will DEcrease contact and therefore friction. A bald or smooth tyre will be best on dry surface. The purpose of tread on tyres is to sipe away water on wet road and a rough tread would act in similar manner though far less efficiently.
Eventually, you will notice that your car is losing traction due to tread loss, especially on wet surfaces. You'll also notice that you may not be getting the mileage out of your car that you used to. A good, simple way to check the tread is with a quarter. On most tires, when the tread is lower than Washington's nose, it's time to take your car in and see of you need new tires.
60% tread wear.
Many tires have tread wear indicator bars molded into the tread. When the tread is worn down to where you can see a solid bar of rubber across the width of the tread, it is time to replace the tire.
A penny can be used to measure for safe tread depth on your car tires. Hold the penny upside-down in a tread groove. If the top of Lincoln's head is visible above the tread wall, the tires are completely worn out, and need to be replaced, pronto.
I assume you are referring to the tread. High grip will have a more aggressive tread design used in off-road conditions or in deep snow. They are called off-road tires or snow tires. The difference is in the tread design.
The "tread" is the part of the tire that is supposed to touch the ground. A tire with no tread would be a tire that is not supposed to touch the ground. You may be wondering why tires have a pattern of grooves (a.k.a., voids) in the tread. They're for bad weather. The grooves allow rain, and (if they're wider) mud and snow to squelch out from under the tire so that the tread can come into contact with something solid on which to get a grip.
so that the car won't slip
Only aggressive tread new tires whine.
Because they have more tread.
the tyres could blow and you may lose controle of your car
Slow down and make sure your tires have at least 2/32" of tread depth.
While it is good to use tires with the same tread and sizes, it is possible to use tires with different tread patterns, though you should be careful as the performance of the two tires may be different, causing your car to be unbalanced, endangering your passengers.