Your tires can affect your car’s performance, and you need them to maintain a reliable grip on the road as you drive, turn, and brake. If you are not happy with the tires you have, you might be wondering if you can get different-sized tires while still using the same rims.
You can change the size of your tires with the same rims, as long as the tire and rims have the same internal diameter. Tires are flexible, however, so the width and height may be changed. Be sure to pay attention to manufacturer specifications when choosing tires.
The key to changing your tire size is to understand how tire specifications work. There’s a system that all tire manufacturers use, and once you understand that, you should be able to find tires that will fit your rims. In this article, I’ll show you how to do just that.
The Tire’s Internal Diameter Must Match the Rim
If you want to change your tire size but use the same rims, the internal diameter of your tires is pretty much set in stone. That means the inside edge of your tires has to match the diameter of your rims.
But the width of your tires does not have to match the width of your rims, at least not precisely.
Either way, the first step to determining whether you can use the same rim will be to get the size of the tires.
Reading Tire Sizes
It can be a bit confusing, but there are three measurements you need to consider when reading the size of your tires.
The first is in millimeters, the second is a percentage, and the last is inches. But once you have them worked out, you should have a good idea of what you are looking for.
You’ll find the size information on the side of the tire, and it will look something like this:
To further complicate things, tire width is measured in millimeters, but rim width is measured in inches.
Here’s what all this means.
- The initial “P” means this is a tire designed for passenger automobiles. Some manufacturers will skip this.
- Next will come the width of the tire, measured in millimeters. This tire will be 250 mm (9.8 in.) wide.
- After the slash is the aspect ratio. This refers to how high the sidewalls are—that’s the rubber between the tread and the inside edge of the tire. This part is a little tricky: the ratio is a percentage of the width. On this tire, the sidewalls will be 55 percent of 250 mm (9.8 in.), which works out to 137.5 mm (5.4 in.).
- The following letter will almost always be an “R.” This refers to how the tire is constructed. Nearly all passenger car tires are radials, which is what the “R” stands for.
- Finally, there’s the inside diameter. This will be measured in inches. This tire’s inside diameter is 17 inches (43.1 cm.).
When looking for new tires, the last number will need to be the same, but you have some flexibility when it comes to the first two numbers, the width and the aspect ratio.
The aspect ratio doesn’t affect how tires fit on a rim, and the sidewalls can flex in or out a bit when you mount the tires, allowing you to experiment with tire width a little bit.
What to Consider Before Changing Tire Size
While wider tires might make for more contact with the road and offer better handling, you don’t want to go overboard. If you go for something too large, you run the risk of damaging your car.
The Size of Wheel Well
Be mindful of how much room your tires will have in the wheel well.
Tires need room to spin, jostle around as they roll over uneven terrain, and turn. As a general rule, there is more room for wider tires than expanding the outside diameter.
You don’t want your tires to rub against the metal in the wheel well or against the brakes or shock absorbers.
Tire Size Affects Your Car’s Performance
Wider tires will also increase friction. That means more road noise and lowered gas mileage.
Changing the outside diameter of your tires will affect how your transmission works, and you can also throw off your speedometer and odometer.
That’s because these were all set for a given tire diameter. If you change the diameter of your tires, you change how far your car travels every time your wheels turn.
The difference may be just an inch or two, but adding five percent to your aspect ratio at highway speeds can add a couple of miles an hour.
Use a tire size calculator to see just how your new tires will affect your speed. If the change is significant, you can get your speedometer adjusted for your new tires.
Why You Might Want to Change Your Tire Size
There are a couple of good reasons why you might want to change the size of your tires, but often it’s a matter of wanting higher performance.
Your tires create four contact patches with the ground. This is literally “where the rubber meets the road.”
The larger those contact patches are, the more your tires will grip the surface when you accelerate, brake, and turn.
There are two ways to expand those contact patches:
- The first is to increase the outer diameter of your tires to make the patches longer, which will increase the aspect ratio.
- The second, and usually the better way, is to find a bit wider tires, making the contact patches wider too.
With larger contact patches, your car will be able to maintain a firmer grip on the road when you accelerate, brake, or make tight turns.
Larger tires are also more appealing. Depending on the make and model, you might find that a larger tire fills up the area between the axle and the wheel wells more, which looks sleeker and more stylish.
For whatever reason, a narrow tire often looks cheap and unattractive.
There are also times when you might opt for a smaller tire. For example, if you install snow tires over the winter, you may be better off with smaller tires.
In heavy snow, smaller tires will make less contact with snowpack, making it easier for you to “slice through” snow and slush. They are also less prone to hydroplaning on wet roads.
What to Look For When Tire Shopping
If you want to try wider tires — or narrower snow tires — remember: you need to make sure the inner diameter matches your rims, but you don’t need to match all the specifications of your old tires.
You might consider trading width for aspect ratio.
That means looking at tires that are a bit wider than your current ones, then shaving a few percentage points off the aspect ratio.
You will want to know the width of your rims. That will be measured in inches, not millimeters, so it should be easy to keep that separate from the tire width.
Once you have found a few sets of tires that you like, look up the range of rims that the different tires will fit on. The manufacturer should have these recommendations handy.
There’s usually a “normal” size rim, but there will also be a range of rim sizes that a given tire will fit on. If your rim width is in that range, you should be able to make those tires fit on your rims.
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Getting the right size tire can be a little complicated, but it’s easy to find tires that work best with your car and driving style once you figure out the system.
Remember that the inside diameter of your tires must match the diameter of your rims exactly—but you have some flexibility regarding the width of your tires and the “aspect ratio” of the side walls.
Check the manufacturer’s recommendations for the make and model of the tire. Each tire has a range of rims they will fit. If your rims fit in that range, the tire should work!
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