Here is an article that sums up, why you should check your tyres.
As your tyres wear and tread depth decreases, your grip reduces and stopping distances increase in wet conditions.
1.5mm is the legal minimum tread depth – any less and you won’t pass a WoF inspection. However, grip in wet conditions reduces significantly if your tread depth gets below 3mm. So it’s best to replace them before you get to 1.5mm.
Remember, the deeper the tread, the safer you’ll be.
An easy way to check your tread depth is to insert a 20c coin into one of the tyre grooves that’s near the middle of the tyre. The base of the number 20 is approximately 2mm from the edge of the coin, so if you can see the whole of the number, it’s time to think about replacing your tyres.
Most tyres have moulded tread-depth indicators which are flush with the tyre tread when it has reached the minimum depth.
Look for a triangle ∆ or TWI (tyre wear indicator) marking which appears in about six places around the side of the tyre.
While you’re at it, check for any bulges on the tyre wall, or objects sticking out that might cause an air leak.
As tyres lose pressure, their safety, handling and fuel efficiency are adversely affected.
When a tyre is under-inflated, its contact patch with the road surface is concentrated towards the two outer edges of the tread, leading to rapid wear on the shoulders and reduced tyre life. Under-inflation also increases rolling resistance, meaning that more energy is needed to turn the wheel – meaning increased fuel use. And under-inflation is the main cause of cracks which could cause blowout and lead to the loss of control of your vehicle.
Tyres lose pressure naturally all the time, so it’s important to check you tyres’ pressure monthly. Make sure you’re using the right pressure for your car – different cars need different tyre pressure. Even on the same car, sometimes tyre pressure is different for the front and the rear tyres.
Step 1) Find the correct air pressure for your car
To find your car’s correct tyre pressure, check the inside of the passenger door or look online at www.energywise.govt.nz/tools/tyre-pressure. You should also be able to find it in the vehicle handbook. If you can’t find your car’s correct tyre pressure, email 
and we will look to see if a match can be made.
Most people use the air pump at their local service station to pump their tyres, however you can also buy air pumps from many automotive retailers.
Step 2) find the valve cap, unscrew and remove it to expose the valve stem
Remove your tyre’s valve cap. Enter your car’s tyres’ correct pressure into the air pump then press its tyre gauge onto the valve stem of the tyre – it will keep inflating if the tyres aren’t inflated enough and will beep if the target pressure has been reached.
Screw the valve cap back on the tyre, and repeat with all your tyres (and don’t forget to check your spare tyre, if you have one). Remember to check your tyre pressure when the tyres are cold (i.e. they have been driven less than 3km).
Bubbles and cracks in the surface of your tyre are usually signs that it’s damaged and is at risk of blowing out. If you have an object sticking out of your tyre it is probably punctured and may go flat, meaning you’re also at risk of a tyre blowing out.
Look for cracks, bulges or bubbles on the sides of your tyre, as well as looking for any sharp objects sticking out or embedded in the tyre. If you discover cracks, bubbles or bulges, it’s time to get the tyre checked over by a professional.
Examples of a bulge and crack on a tyre sidewall
If you discover any sharp objects, don’t be tempted to pull them out yourself as this may lead to a sudden loss of pressure, and render the tyre useless. Drop into your local tyre retailer, who’ll be able to advice on the best action to take. In some instances, the tyre can be repaired, however in others the tyre may need to be replaced.
If your tread depth is low or your tyre is unrepairable, you’ll need to get new tyres. As a rule of thumb, cars need new tyres about every three years but this does vary. If you drive a lot, they’ll probably need to be replaced more often. If you find yourself driving in wet conditions a lot, you’ll also probably want to replace them more often (to ensure good wet grip is maintained).
The most important features to look for are good wet grip braking, and fuel efficiency (sometimes called ‘low rolling resistance’). Some tyres are also designed to make less noise. Buy the best tyres you can afford within your budget. Ask your tyre retailer for more information.