What is the Biggest Tire for an 18 Inch Rim?


If you have a car or truck that has 18-inch rims, it’s important to find tires that will properly fit your vehicle. 

If you are going to choose the biggest tires for your rims, be sure to understand the difference between standard passenger tires and high floatation tires, since the size guide will vary.

When choosing the largest standard passenger-sized tire, find a tire labeled as “325/65R18.” The largest tires for high floatation tires are labeled as “35X12.5R18.” It is important to find tires that fit well with the vehicle and aren’t too big, otherwise, the tires may cause issues on the vehicle.

It’s important to know which tire size is right for your vehicle so you can have a comfortable and aesthetically pleasing ride.

How to Read 18 Inch Rim Tire Sizes

Passenger Car and Light Truck Vehicle Tire Sizes

Before we move on to how big your tires should be, it’s important to go over how to read the different tire sizes. Let’s take 325/65R18 for example. 

What do all those numbers and letters truly mean?

The first number, 325, is the width of the tires in millimeters. 

The 65 in the example above is the aspect ratio. The aspect ratio is the height of the sidewall. 

This measurement is taken from the rim of the wheel all the way to the height of the tread. 

The aspect ratio compares the sidewall height to the width of the tire, so in this example, 65% of the tire is as high as the tire is wide.

The R right next to the number 65 is the way that the tire was constructed. 

There are 3 different ways that a tire can be constructed, including:

  • Radial (R)
  • Bias Belt (B)
  • Diagonal (D)

Most of the time, tires are constructed radially. 

This type of tire is standard for the industry today because it creates a more comfortable ride, gets better gas mileage, and is more durable.

The number that follows the construction type is the rim diameter. 

In the example above, the number is 18. 

This is very important when you are looking for brand new tires to ensure that the new tires will actually fit around your wheels.

It’s important to mention that some passenger tires have a few more additions to their tire labels. 

For example, the tire label might read P325/65R18 94H. 

The P in the front stands for Passenger vehicle, and other variations include LT (light truck), T (temporary spare designation), and C (commercial).

The numbers after the rim size stand for the service descriptions for the tire. 

The 94 in the second example above is the tire load, and the letter stands for the speed symbol or tire load index.

High Floatation Tire Sizes

If you are going to use high floatation tires, then the label will look a bit different. The biggest size available for this type of tire is 35X12.5R18, and the smallest size is 33X10.5R18. So let’s break these numbers and letters down.

The first number, 35, is the diameter size of the overall wheel.

The following number after the X is the width of the tire, which is 12.5 in this case. 

The R is for the same purpose of the passenger car and light truck tire sizes; it just stands for how the tire was constructed. The number that follows the construction type is the size of the rim.

Passenger Car and Light Truck Vehicle Tires vs. High Floatation Tires

The sizes of passenger car tires range from 205/40R18 all the way to 325/65R18. 

These are great standard tires that can work for smaller trucks, SUVs, and crossover SUVs

If anything, these types of tires are found on standard vehicles such as SUVs, lighter trucks, and sedans.

If you work in industrial or farming occupations, it is best to get high flotation wheels for your vehicle. 

The main difference between passenger car tires and high floatation tires is the way that these types of tires “float” in muddy and sandy terrain. 

These types of tires easily glide over loose sediment, which is why they are often used in farming.

Whether you want standard passenger tires or high floatation tires comes down to your lifestyle and preference. 

If you know that you are going to be using your vehicle for commuting rather than work, you might want the passenger tires. 

If you are going to be using your truck for work, towing, and off-roading, it would be best to purchase the high floatation tires.

How Big of Tires Should Be Purchased for Your Vehicle

When picking out new tires for your vehicle, it’s very important that you know the make and the model of your car. 

If you choose a tire size that is too big, then you can run the risk of having the tire too close to other objects in your wheel well. 

If you get tires that are too small, there can be speedometer errors and transmission issues.

If you are going to purchase large tires, just make sure that you have a minimum of 1/4″ to 1/2″ of space between your tire and anything within the wheel well

So, if you want to purchase the biggest tires possible for 18-inch rims, choose 325/65R18 for passenger tires or 35X12.5R18 tires depending on what type of vehicle you drive.

Benefits and Drawbacks of 18 Inch Rims and Tires

Choosing large wheels for your vehicle has both its pros and cons. 

If you do choose the largest tires for your 18-inch rims, here are some things to look forward to:

  • Comfortable Ride
  • More cushion and padding
  • Increased ground clearance
  • Better for off-roading and four-wheel driving
  • Good handling
  • Large rims give the vehicle a more aesthetic look

Of course, there are some drawbacks when you choose to put larger wheels and tires on your vehicle, including:

  • Worse fuel economy
  • Need to change your vehicle’s suspension
  • New rims can be quite expensive
  • Doesn’t allow the vehicle to turn as sharply
  • Doesn’t ride well on highways
  • Louder tires on roads due to deep tread

With all the benefits and drawbacks to getting the largest tires on your vehicle, there are definite trade-offs. 

Ultimately, you should decide whether getting large tires will be an added benefit or a hindrance to your vehicle. 

Will Turner

Will has an absolute passion for 4x4s and loves discovering all of the small details about each model. Will joined the Four Wheel Trends team in early 2021 and has been a valuable contributor ever since!

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