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Tire size is a pretty complicated subject since there are so many numbers involved. But once you get the gist of tire sizing, it becomes pretty easy to understand.
There are two ways to measure tires. The first measuring method is metric and the second method is floatation. Metric tire sizes start with the numbers in millimeters and there are four different categories of metric tire sizes: Passenger (P), Light Truck (LT), Special Trailer (ST), and Temporary (T).
The class of tire is usually imprinted on the tire before the tire size. For example, LT285/75/16. The first number on the tire size is the section width of a tire. This is a measurement of the width of the tire, basically from the sidewall to the sidewall.
The next number in a tire size is a percentage and is the percentage of the width of the tire. It’s a number that’s used to determine the height of the tire’s sidewall. And the last number is the tire diameter or the size of the wheel. The wheel diameter is usually separated by an “R”, meaning the tire is “radial ply” in comparison with bias ply.
Radial tires are the most common type of tire on the market today. Light truck tires have additional plies of woven nylon steel. These additional layers or plies construct what is known as the carcass of the tires. The more plies a tire has the stronger a tire is and has more capacity to hold a higher air pressure.
Now with metric sizes out of the way, let’s look at what flotation sizes are. Flotation sizes directly relate to the tire’s size in inches. The first number in flotation sizes is the height of the tires in inches, the second number is the width of the tire in inches, and the third number is the wheel size or the rim diameter.
For example, if you have a 35 inch by 12.50, 17. This means the height of the tire is 35 inches, 12 and a half inches wide, and fits a 17 inch wheel. So when someone says they have 33 inch tires they are referring to flotation sizes.
What Are 33’s
When someone says they have 33’s, they are referring to the overall diameter of the tire. This means the rim diameter plus the width of a sidewall. In flotation sizes, this is measured in inches compared to metric which is measured in millimeters and percentages.
Flotation sizes and their accepted metric equivalent size can be confusing but both numbers should be stamped into the sidewall of a tire. In tire shops, the more modern custom is to use metric sizes.
But metric sizes don’t take into account the actual diameter of the tire. They just give the percentage of the tread width which can be confusing. Both measurement systems do take the rim diameters into account. But metric size tires go about it in a more abstract way.
So when someone says they have 33’s, it’s not exactly clear as to what that means because 33’s can fit onto a number of different wheel sizes. For example, 33’s can fit onto a 16, 17, 18, or a 19 inch rim. This changes the sidewall height, but does not change the overall tire height.
The metric system would measure each of these tires differently as well. For example, the percentage of the sidewall height would decrease as the width of the rim increases.
When someone says they have 33 inch tires on their vehicle, it’s basically saying they have 75% of 285mm’s on their vehicle. It’s just that 33’s are easier to compute. So 33’s come in many different forms.
Listing out all the possible sizes of 33’s would be endless since there are so many factors to take into account such as wheel rim surface, rim width, whether the vehicle has a lift kit, and radial construction of the tire.
But one thing you can be sure of is that 33’s belong on light trucks. The 33 inch tire is usually not found on 4 door sedans. They are most commonly used on off road vehicles such as SUVs and trucks. These tires are built to handle rough terrains.
If you are unsure of what size tire to get for your vehicle you can usually find the information in your vehicle’s owner manual. Or you can get the size of the tire information from the tires that are currently placed on your tires.
Tire speed ratings are ratings for your tire that recommend the max speed of a tire. All tires are meant to go the same speed as each other. If you exceed the speed rating on some tires, you risk the chance of tire deterioration and blowouts.
Tire speed ratings refer to the maximum speed rating a tire can go. Tire speed ratings are usually displayed on the side of a tire in the form of a letter such as “S” or “H”. This number is stamped into the sidewall of a tire.
Tire speed rating is figured out by the manufacturer through a series of tests. The tests put the tires through different stressors and see how well the tire holds up under that speed. If the tire starts to fail while going above a certain speed rating, its rating is dropped down.
The “S” in a 33S tire means that the tire’s speed rating is 112 mph. This means that a 33 inch tire can reach up to 112 miles per hour without failing. That’s pretty fast for a large tire but these tires are constructed with extra plies so the speed rating should not shock you.
This doesn’t mean the tire will automatically fail if you go above 112 mph. It just means that it’s recommended you don’t go above that speed rating for long periods of time while driving on that tire.
About The Author
Christopher Sparks has been servicing vehicles since 2012. After completing the automotive studies program at Camden County College, he was awarded an Associates's Degree in Applied Science. His first job was a lube-tech at Jiffy Lube, and is currently an independent B-Technician servicing vehicles for the United States Postal Service. Christopher is ASE certified and loves rebuilding engines.Read more about Christopher Sparks