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Become a wheels and tires pro

7 min reading time
To change or not to change? Learn why you really should switch to winter tires when it gets cold. We also have answers to other questions about your wheels and tires, like: What does tire balancing mean?

27 September 2019

This fact is an indicator of how important your car tires are for road safety: The surface contact of all four tires combined is about the size of a piece of letter paper. This contact with the road is where everything happens – accelerating, braking and directional control. This is why you should take your tires seriously. Read on to find out everything you need to know.

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You can also listen to this article via Changing Lanes, the official BMW podcast. Apart from this and other narrated articles, Changing Lanes offers you fresh new episodes every week, packed with exclusive insights on tech, lifestyle, design, cars, and more – brought to you by hosts Sara and Jonathan. Find and subscribe to Changing Lanes on all major podcasting platforms.

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Don’t drive on summer tires in the winter.Don’t drive on summer tires in the winter.
Driving on summer tires in the winter is a really bad idea.

1. Do I need winter tires?

Why not just drive with your winter tires in summer? Then you don't have to pay for two sets of tires and for changing them. The reason is simply that it’s dangerous – you're putting your safety, that of your passengers and other drivers at risk. Tires are a major factor in how safe your car is to drive. So what's the difference between summer tires and winter tires then?

The rubber compound

The amount of rubber used in winter tires helps the tire stay soft and flexible so it can grip the road when it's cold outside. If you drive on winter tires in the summer, then your tires will be too soft, which means they will wear faster, reduce fuel efficiency and also need a greater distance for braking. The reason for this is because winter tires are more pliable at higher temperatures, so they wear more quickly on hard, dry asphalt.

The rubber compound used in summer tires is considerably harder than it is in winter tires so they can handle the heat of summer. If you drive in the winter with summer tires, stopping distances will be longer and it will be harder to drive in a straight line because the tires aren't soft enough to grip the road. If you're even able to get going, that is…

Tire tread

Summer tires have large contact patches which give the car a better grip on the road. Although the tread pattern has fewer grooves and sipes (thin slits that cut across the rubber) than that of winter tires, the grooves are bigger so they can move large quantities of water away to the sides, maximizing the contact with the road in order to avoid hydroplaning.

Winter tires, on the other hand, have a lot of grooves, which are also deeper than those in summer tires. It is these grooves that allow winter tires to keep their traction on snow and ice. There are also smaller channels called sipes that help the grooves keep the tire in contact with the surface.

Your car will skid if you drive on summer tires in winter – see for yourself what that looks like and how winter tires fare in the summer.


2. When should I change my tires?

Experts agree: Winter tires are just for winter. But when should you change your tires? Whenever the temperature consistently stays below 50 degrees is when you need to use your winter tires. The rule of thumb is that below 50 degrees is when winter tires do best on the road and above 50 degrees is when summer tires do best.

Whether the use of winter tires is obligatory  depends on which country you live in. The US and most of Canada don’t have any tire laws requiring winter tires. But the Canadian province of Quebec does, so be sure to read up on it if you plan to drive there in winter.

wheels and tireswheels and tires
Should you change your tires? Absolutely! But when?

3. When should I buy new car tires?

In general, you should get new tires when your tread is worn down. The legal limit for tread depth is 2/32 of an inch, but experts recommend about 5/32 of an inch for winter tires and 4/32 of an inch for summer tires.

So how do you know if you have enough tread or not? An easy way to check the tire tread depth is to use the penny test. Insert the penny into your tire’s tread groove with Lincoln’s head upside down, facing you. Check several grooves on your tire, but especially those on the outside where the tread wears the fastest. If you can see all of Lincoln’s head, you have less than 2/32 inch remaining and it’s time to replace your tires.

A lot of drivers don't know that tires age even if they aren't being used. UV rays, humidity and temperature all degrade the material. This is why you should buy new tires every eight years even if you have plenty of tread left.

Each time you change the wheels or tires and have driven about 50 miles, you should retighten the lug nuts on the wheel rims. This is purely a precautionary measure, but under certain circumstances it is possible for the nuts to loosen up a bit during daily use.

alloy wheels tire labelling tire size explainedalloy wheels tire labelling tire size explained
Tire size explained. All tires have certain numbers and letters on the side.

4. How do I read a tire sidewall?

P245/40 R19 98V - even though it may look like one, those numbers on your car's sidewall are not a secret code. They are known as the “tire code”.

The first part of these numbers (P245/40 R19) tells you the tire size:

  • The letter “P” at the beginning tells you it is a P-metric tire made to standards in the United States and intended for passenger vehicles. If a tire size has no letters at the beginning, then it is a Euro-metric tire constructed according to European standards.
  • The first three-digit number in the tire size is the tire width. So in this case, the width of the tire from sidewall to sidewall is 245 millimeters.
  • The 40 in this tire size tells you the aspect ratio, which means that the height is equal to 40 percent of the tire's width.
  • The letter “R” stands for “radial” because the layers run radially across the tire.
  • The 19 refers to the wheel diameter, which is the size of the wheel that the tire is intended to fit, i.e. this tire is made for a wheel with a 19" diameter.
  • The last part of the code (98V) is the load index and speed rating.
  • 98 refers to the load index, or how much weight the tire can support when it’s properly inflated. If you consult a tire road index chart, you will find that the 98 means the tire can carry 1,653 pounds.
  • The final “V” at the end is a speed rating, indicating the maximum speed this particular tire can sustain under its recommended load capacity. Ratings range from A to Z, and in this case, V is equivalent to a maximum speed of 149 mph.

Only certified winter tires can carry the snowflake symbol, while the M+S (mud and snow) following it can often be found on all-weather tires as well. The Department of Transportation number on the tire indicates when the tire was made. For instance, 2519 means that the tire was produced in the 25th week of 2019.

performance tires tire tread depthperformance tires tire tread depth
All-season tires or specialty tires?

5. What are the different types of tires?

If you live in the south where warm winters are common and snow is all too rare, then using all-weather tires year round will likely be fine. All-weather tires (or all-season tires) are halfway between summer tires and winter tires. Basically, all-weather tires are winter tires that have been given some aspects of summer tires, which means they are always a compromise between the two.

Other types of car tires:

  • Low rolling resistance tires: This type of tire reduces resistance so you save on gas or electricity. The rubber compound used is different to that of regular tires, so the tread is smoother, meaning these tires have less grip and offer less comfort than comparable tires.
  • M+S stands for mud and snow and is often found on all-weather tires.
  • Offroad tires have a lot more rubber, which makes them a lot better on unpaved surfaces. But then, of course this means they do not perform as well on paved surfaces.
  • Runflat tires let you keep on driving even after your tire has been punctured, but only up to 50 miles at a maximum of 50 mph. And they should really only be installed on cars that have a tire-pressure monitoring system and are approved for this type of tire because otherwise you might not know you have a flat.
  • Racing slicks have very little little to no tread pattern. This is to make sure that as much of the tire as possible is hitting the ground. While they are not at all meant for use in the rain, under ideal track conditions, they provide optimum traction. However, most of these tires are not approved for street use.
  • DOT R compound tires are basically racing tires that have been created to comply with DOT requirements. They do have grooves, but they’re nothing like production tires. Although R compounds are street legal, they are terrible for driving on wet road pavement.
  • Studded snow tires are winter tires with metal studs that chip into the ice to create traction. They are not permitted in some states in the US, so you should check if you're planning to drive to other states in winter.

6. What should my tire pressure be?

The correct air pressure is determined by car manufacturers and tire makers. You can consult a tire pressure chart – usually located in the car's door jamb or sometimes in the trunk, but always in your owner's manual – to find the right tire pressure for your tires. Tire pressure depends on the type of vehicle you have, the type of tire and the load. You should check your tire pressure regularly, especially before you take any long trips.

7. What are directional tread patterns?

Certain car tires have a directional tread pattern, which channels water away and increases stability. These tires have arrows on them pointing in the direction that the tire needs to be mounted. When you change your tires, make sure they are all mounted in the right direction. If they are mounted incorrectly, it will result in a much noisier ride and they will wear faster.

Storing your car tiresStoring your car tires
Storing your tires: Cool, dry and dark is the way to go.

8. What is the correct way to store tires?

After changing the entire tire (tire with the rim), they should be stored horizontally and on top of one another. You should store tires without the rim standing up, and turn them from time to time. For both options, the same conditions should be observed: as dry, cool and dark as possible. The way you store your tires affects the lifespan of the rubber compound.

winter tires winter tireswinter tires winter tires
Not ideal: Winter tires are not the best for driving when it's hot outside.

9. What does tire balancing mean?

How do you know if your tires are unbalanced? One obvious sign is when your steering wheel starts to vibrate. A car repair shop should have the right tools to check the balance of your tires and adjust them. To rebalance the tire, weights are applied to planes on the tire rim, inside and out.

10. What is hydroplaning?

Hydroplaning is something everyone worries about. It happens when there are large amounts of water on the road that the tires aren't able to displace. Water is pushed under the tire, creating a thin film that separates the tire from the road surface, causing it to lose traction. This results in a loss of steering and braking ability. Tires more likely to hydroplane are ones that are especially wide (they have more water to push away) and tires with a worn tread. When there is little tread left, the grooves fill with water so the tire can't displace the water. The same thing can happen if the tire pressure isn’t right.

What should you do if you suddenly find yourself hyrdoplaning? Take your foot off the gas, but do not brake. Avoid steering and disengage the clutch if your car is a standard. Wait until you feel the tires reconnect with the surface of the road.

“Safety and style are in the detail.”“Safety and style are in the detail.”

“Safety and style are in the detail.”

Take control. With original BMW winter wheels and tires.

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