We all love our cars, and we tend to care for them. A healthy car will keep running for longer, so regular maintenance is the best way to do that. On top of that, there are some areas that should be regularly maintained because they are directly related to safety.
In this aspect, one of the main components is the tires. The black rubber wrapped around the wheels is the only piece of the car that is touching the road, so they help it accelerate brake and change direction.
In many cases, car owners don’t bother too much when it comes to the size of the tires. They know the size that comes from the factory, and they purchase every new set with the same dimensions. Unlike them, there are people like me that want to change things up for various reasons. On the one hand, there is the aesthetic side of things, and on the other, there is the performance.
Making changes to the wheel and tire size can positively impact how your car drives and handles. With that said, if you get it wrong, you may have a bad time and a car that won’t handle better and, in some cases, even worse.
This may seem trivial, but getting things right is crucial, so today, I’ll guide you through the process and explain everything you need to know about making these changes.
Wheel and Tire Setup
I’m guessing that non-car guys know the numbers they need to know for the tires and wheels but don’t know what they mean. Before I begin explaining about changing the size, first, let me explain what everything means.
Let’s start with the tire size, which is something that you most commonly need. It consists of 3 parts that basically determine the dimensions. The tires I use on my Corolla are P175/70R13; yes, 13-inch wheels still exist.
So, the first number, “175,” determines the tire’s width, and it’s measured in millimeters. Next up is the sidewall, which is the second number and is calculated as a percentage. In my case, the “70” means that the sidewall height is 70% from the tire’s width, meaning that my sidewall is 122.5 millimeters. As you may have guessed, the last number, “13,” is the rim’s size, basically the cutout in the middle of the tire where the wheel goes. Interestingly, you have a combination of imperial and metric dimensions, plus a percentage to keep things as confusing as possible.
Don’t think I forgot about the letters. In the example, the size begins with the letter “P,” which stands for passenger tire and is something you’d find on hatchbacks, sedans, crossovers, etc. There are also “LT” and “ST,” where the first one is Light Truck, a tire capable of heavier loads, while “ST” stands for Special Trailer and, you guessed it, it’s used for trailers.
Before the diameter, there is an “R,” which stands for Radial. This determines internal construction on the tire’s casing. While radials are the most commonly used tires today, you can also find B for Belted Bias or D for Diagonal Bias. All 3 are different types of internal construction.
Load and speed rating
An often-overlooked part of tires is the load and speed rating. As the name suggests, these ratings are assigned to tires depending on how much weight they can handle safely and the maximum speed you should be driving them.
My tires are 82T which gives us both ratings. The number determines the load rating, and 82 means that each tire is capable of handling 1047 lbs. of load, which is more than enough, considering the car barely tips the scale over 2000 lbs. As far as the letter goes, “T” in this case means that the tire will be safe to be driven at speeds up to 130 mph. Both the load and speed ratings can be read from a chart and are an industry standard.
|Speed Index||Speed in mph||Speed in km/h|
|L||75 mph||120 km/h|
|M||81 mph||130 km/h|
|P||93 mph||150 km/h|
|Q||99 mph||160 km/h|
|R||106 mph||170 km/h|
|S||112 mph||180 km/h|
|T||118 mph||190 km/h|
|U||124 mph||200 km/h|
|H||130 mph||210 km/h|
|V||149 mph||240 km/h|
|W||168 mph||270 km/h|
|Y||186 mph||300 km/h|
|Load Index||Maximum Weight in Pounds||Maximum Weight in Kilograms||Load Index||Maximum Weight in Pounds||Maximum Weight in Kilograms||Load Index||Maximum Weight in Pounds||Maximum Weight in Kilograms|
Later in this article, I’ll explain why my tire isn’t per the factory specs recommended by Toyota.
With the tires aside, let’s look at the rim size. The wheels on my car are 5jx13 ET39, which gives us several dimensions.
First, the number “5” determines the width of the rim, and in this case, it means that it’s 5 inches wide. The letter “J” is the tire bead profile and is a common one for a passenger car. Depending on the type of vehicle, there are other designations that can be read off the chart, but for passenger cars all rims have a J bead profile. Next up is the “13,” with determines the diameter of the wheel.
In the case of wheels, “ET” doesn’t stand for extra-terrestrial. Instead, it determines the offset of the wheel, which is the distance from the central point of the wheel to the mounting point on the inside. In my case, the offset is 39, meaning that the distance is 39 millimeters, and since it’s a positive number, the mounting point is outward. There are wheels with negative offset, meaning that the mounting point is inward. A negative offset will push the wheel outward, and you can find these wheels in show cars, mostly.
As part of the wheel size is the center bore, which is the big hole in the center of the wheel. It’s designed to be a tight fit for the hub, meaning that it’s something you should pay attention to as well because sometimes you’ll need to dig in the technical details to find it. The unit is in millimeters, and the one I have is 54.1.
The last piece of information you should know is the PCD, which is short for Pitch Circle Diameter. This determines the number of bolts and the distance between them. Manufacturers use different setups depending on how the car is designed, and while today’s standard is 5 or 6 lugs or bolts, there were times with only 3.
Reverting to my Corolla, the PCD is 4×100, meaning that the wheel has 4 bolts, and if you draw a circle using those points, you will get one with a diameter of 100 millimeters.
Making the changes
Now, with all the introduction out of the way, let’s look at what you can and cannot change.
Things you cannot change
The first thing you cannot change is the PCD, meaning that if you decide to change your wheels, the diameter and holes need to be the same. There are ways to change this, but it requires replacing multiple parts.
Next up is the center bore, over which there is a debate going on. Some people say that as long as the hub will fit, you’re good to go. Technically that’s true, and getting wheels with a bigger bore won’t be a problem as far as installing the wheels go. On the other hand, you may experience some faint vibrations, which aren’t something you’d want on your car. My advice is to go with the bore diameter from the original wheels.
Finally, the load and speed rating. Technically, these can be changed, and I am living proof that it’s possible, but I wouldn’t recommend it. Remember where I said that my tire isn’t per spec? Well, this is it. Toyota says that my tire should be 81H, but my tire is 82T. This means that the ones I have installed are rated for slightly higher load but are for lower speeds. 130 mph is a speed that’s far higher than I’ve ever driven my car, so this isn’t an unsafe approach. Going above the factory rating also won’t be a problem, and the tire will remain safe, Despite all of this, the load and speed ratings of a tire should be per the manufacturer’s requirements.
Things you can change
Everything that isn’t mentioned above can be changed, but you should be careful in doing that.
The most common thing that gets changed is the tires, which I did a long time ago. From the factory, my car came with a tire that was 155 wide and looked like it was running on bicycle tires. Changing the width by 20 millimeters gave it a bit of a grownup look, and I have no complaints there.
When you change the tire’s width, there is an important thing you should keep in mind. A tire can be as wide as the rim, so these two dimensions are closely related. The width of the rim will determine the minimum and maximum width of the tire you can have installed on your car. Looking at the chart, in my situation, the minimum width that can be fitted to a 5-inch wheel is 155, and the maximum is 185. So if I were to put 195 or 205 wide tires, I’d need to replace the wheels.
|Rim Width||Tire’s Min Width||Tire’s Max Width|
|5,0 Inches||155 mm||185 mm|
|5,5 Inches||165 mm||195 mm|
|6,0 Inches||175 mm||205 mm|
|6,5 Inches||185 mm||215 mm|
|7,0 Inches||195 mm||225 mm|
|7,5 Inches||205 mm||235 mm|
|8,0 Inches||215 mm||245 mm|
|8,5 Inches||225 mm||255 mm|
|9,0 Inches||235 mm||265 mm|
|9,5 Inches||245 mm||275 mm|
|10,0 Inches||255 mm||285 mm|
|10,5 Inches||265 mm||295 mm|
|11,0 Inches||275 mm||305 mm|
|11,5 Inches||285 mm||315 mm|
|12,0 Inches||295 mm||325 mm|
|12,5 Inches||305 mm||335 mm|
While I’m on the subject of wheel width, let’s look at that. For the sake of this example, let’s say you want to change the width of the rim. There are several things you should check before doing so.
Widening the rim means that there will be less space between the inside of it and the suspension components or the plastic in the wheel well. On the outside, you will be reducing the space between the arch and the tire. In both cases, you are risking contact, leading to some serious complications. If you don’t go overboard, things should be fine, but just to be sure, check the distance between the tire and the inside of the wheel well when you have a full steering lock. This should give you an idea of how much breathing room you have. For the arch, there isn’t an easy way to determine this apart from going to a specialist. Most people don’t do this, and you hear some complaining about the tire rubbing on the arch when they hit a pothole.
Changing the width of the rim will undoubtedly result in rethinking the offset. Sticking with the stock offset may be fine if you’re increasing the width by half an inch, but for more than that, you’ll probably need to make changes. If I were to get a rim with 39 offset and 6 inches wide, it means that it should add half an inch to both sides of the current wheels. If that happens, I’ll definitely have some rubbing on the arch, so I’d need to push the wheel a bit inward, which can be achieved by going for a rim with a bigger positive offset.
One thing that the width and offset may change is the scrub radius. This is the area that usually flies under the radar, and many people neglect it, but it’s essential in terms of handling and stability. The scrub radius is determined by the point where the line of the suspension and the middle of the wheel meet. Naturally, you’d have to draw an imaginary line for this, but every car has it because the suspension is at an angle, and there will be a point where both will cross paths. As a reference point, you take the one where the tire touches the road, and if the scrub radius point is below that, then it’s positive, if it’s above that, it’s negative, while it is negative if it’s on the same spot. Both the negative and positive scrub radiuses have pros and cons. Each car is set up differently from the factory, so try not to mess too much if you don’t want to change how it drives. If you’re not too sure about this, I’d suggest consulting with a professional.
What about the diameter? You can increase it, but not only will you need new tires, there are also several things to take into consideration. The first problem is the obvious one, the size of the wheel may not fit the well if you decide to go too big. Increasing the size by an inch or two should be fine in most cases, but for more than that, you may risk some fitment issues.
Replacing the tires if you change the diameter is one thing, but during that process, you’ll need to think about the total diameter of the wheel and the tire. As the tires rotate, the make a certain amount of revolutions per mile. Changing the diameter means that the number of revolutions changes, which will cause problems with how the dashboard reads the speed and distance. The instruments are fine-tuned to show both units precisely based on the number of revolutions, so changing them will give you incorrect readings.
With a bit of math, I have determined that the total diameter of my wheels and tires is 22.6 inches, and with that, each tire makes 891 revolutions per mile. To remain in the ballpark, if I were to increase my rims from 13 to 15 inches, I’d need to decrease the size of the sidewall. For example, from 175/70R13, I’d need to go to something like 175/55R15, which gives me the same total diameter. With a slight discrepancy, I can even change the width and go with something like 195/50R15, which will increase the total diameter by 0.4%, which may be considered negligible.
Wrapping things up
If you started off reading this article thinking that wheel upgrade is simple, you probably think that it’s rocket science. It’s not rocket science, but it is car science. Manufacturers spend billions on research and development, making the cars the way they are, so for us to make any changes means that we may mess it up. The good news is that there are ways to avoid making a complete mess and still get a car that drives well. The good news is that if you know what you’re doing, you can make the car drive better, but in a way that’s better suited for you.
As I mentioned several times throughout the guide: If you’re not sure about something, consult a professional. It may cost, but you can be sure that you won’t be making a terrible mistake that can be costly but also unsafe.
Can I put smaller wheels on my car?
Of course. The rules for increasing the wheel size are the same if you want to decrease it. As long as everything fits without scrubbing and the total diameter remains unchanged, you should be good to go.
Are wider wheels better?
It depends. A wider rim means that you’ll need to fit wider tires, which will result in a wider contact patch. As a result, you should get increased traction but a tad worse aquaplaning resistance. With that, going for a lot wider tire will result in worse gas mileage.
Is it better to have a bigger or smaller rim?
In both cases, you are looking at some pros and cons, so it’s up to you to decide which one is better suited for you. Going for a smaller rim means that you can fit a tire with a bigger sidewall, which is a good thing if you’re after comfort. The increase in sidewall height means that there will be more travel when you hit a hole and result in a softer ride. On the negative side, you are looking at a setup that won’t be too dynamically inclined, as the tire will be slightly softer and the sidewall will flex. Increasing the diameter means that you’ll need tires with thinner sidewall, giving you a more dynamic driving feel and less sidewall flex but at the same time a bit harsher ride.
Can I replace steel wheels with alloys?
Yes. The material of the wheels is irrelevant in this case. Regardless of what a wheel is made of, whoever made it made sure that it’s designed with a specific dimension in mind, meaning that anyone can install it as long as it fits on their car.