Changing Tire Size Guide
Whenever we’re talking about enthusiasts, the first thing that comes to mind is modifications. We’re constantly looking for ways to visually of functionally improve our cars. Making the right modifications is important since they differ from what our vehicles came from.
None of us will assume we’re smarter than engineers, but some modifications we make match our needs better. Regardless if we’re looking for visual improvements or driving characteristics, we need to be very mindful of any modifications we’re trying to make.
Tire size is one of the most common options here. Regardless if we’re looking for tires with lower profile tires, or want to increase the diameter to have more flexible options, getting the right size for your car is essential.
This guide will help you if you want to freshen things up. I’ll outline the important things you should consider when changing the tire size and how to do it properly.
Things you shouldn’t change with tires
This isn’t technically a tire size aspect, but I have to mention it because it’s directly related to safety. If you’ve read my tire size guide, you already know all the various aspects of it. Among the many things, there are several you cannot change and those are the speed rating and load index.
Technically, we can change these, and I am living proof that it’s possible, but I wouldn’t recommend it. My Corolla, for example, isn’t per factory specs. 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 it’s not the worst thing in the world. With that said, it’s still unsafe, which is the main reason I’m not recommending this approach.
How I use the car is the most important thing here. I don’t drive it too much these days and it only sees city driving at speeds that are significantly lower than the ones that the tires are rated at. I’m not justifying my actions, I’m trying to explain that it’s not the worst thing I’ve done in my life.
Going above the factory rating won’t be a problem, and the tire will remain safe. The tires will cope with higher speeds than the manufacturers specified, so there’s no risk if you don’t consider the price difference. In some special cases, you can go higher because you may not find the exact match if you’re changing the tire size.
Despite all of this, the load and speed ratings of a tire should be per the manufacturer’s requirements.
Things you can change with tires
As far as changing is concerned, any kind of modifications related to the size is fine, as long as you’re “following” the important rules. Regardless of the model we’re talking about, you can change the width, side profile, and diameter. Keep in mind that you’ll need to aim for tire dimensions manufacturers produce. Most of us would love to pay for custom dimensions, but that’s not how things work.
How does the process start? The total diameter of the tire is the first thing you need to consider. This is how the car is set up from the factory, and based on the diameter, the manufacturer has the speedometer set up to give a correct readout. The total diameter is also important to ensure that the tires fit in the wheel wells.
To give you a better understanding of this, let’s look at my Corolla. The car came from the factory with 155/80R13, and it looked like it rode on bicycle tires with a larger sidewall than a tractor. Among the first modifications the car had was the tire size.
I went with the wider option of 175, which meant that leaving the profile identical to the previous size would have caused problems. Fitting 175/80R13 in the wheel wells of the car isn’t a problem, but that would have meant that I would need to make some changes to the speedometer. The readings will be off because a tire with a larger diameter has fewer revolutions per minute.
To compensate for the wider tire, I needed to reduce the sidewall percentage to get a similar diameter. As a result, the tires that are currently on the car are 175/70R13, so 20mm wider tire and 10 percent less sidewall. It’s not identical to the OE dimension, but it’s close enough.
What if I would want to fit tires with larger wheels, let’s say 15-inch ones? This involves a few more changes. Leaving the width the same as it is now, I would have something like 175/70R15, a dimension which I’m not sure it exists. I did the research and a 15-inch model with 195 width exists, so that’s my goal.
Doing this will change the diameter by 2 inches, so I’ll have a lot fewer revolutions per minute, so the speedometer will be way off. Like in the previous example, I’d need to change the sidewall to ensure that the total diameter is identical or very close to the original one.
As a result, if I were to get 15-inch tires for my Corolla, the dimension I should aim for is 195/50R15. It’s a lot wider tire than the original one and the profile is lower. The ride quality will have an impact, but I should gain some coolness and sharper handling.
I used a specific example to give you an idea of how you should think when you’re changing the tire size. There are multiple aspects you should consider, revolving around the fitment and the total diameter of the tire. In addition to that, you shouldn’t ignore the correlation between the size of the wheels and the tires.
From my wheel size upgrade guide, you can see the table in which I outlined the minimum and maximum width of a tire that can be fitted. In my situation, 5-inch-wide wheels aren’t ideal for 195 wide tires, so I’ll need to upgrade that part as well.
A good rule of thumb about the sizes you can fit is to see what the manufacturer specified from the factory. My Corolla came with the 1.3 carburetted engine, so it’s the lower tier model. The most I can go is 14 inches and 185 wide tires, which is fine, but I’m looking for a more modern look, which is why I’m aiming for 15 inch models.
The best way to get the numbers right is to check out websites that have tire diameter calculators or do the math yourself. If you have the original dimensions, you can use either way.
Upgrading the tire size of your car doesn’t have to be a troublesome process. Sure, there is some thinking involved, but at the end of the day, if you’re not going overboard with it, you should be fine. There are several aspects to keep in mind and as long as you don’t ignore them, you should be fine.
In a worst-case scenario where you don’t know where to start, but you’re itching to make the upgrade, talk to a professional. You can reach out to us with a specific requirement and we’ll try to give you the best answer possible. It may be theoretical, and there are some things that are unknown to us, but in a best-case scenario, you’ll have a good starting point.