- A short note here: As a former Bridgestone engineer, I strongly recommend checking Discount Tire - Treadwell out. Now, you can effortlessly get recommendations based on value, performance, and local favorites without going to the Discount Tire store.
TRIED & TRUSTED!
Tire size contains tons of digits and letters. Moreover, some of these numbers aren’t the same unit. In a word, it’s confusing. Though, no worries, you gonna learn it from a former Bridgestone engineer. Let’s begin!
How does Tire Size Work?
Tire size is the numbers and letters that are indicated on the tire sidewall. These digits and letters remark,
- Tire Type
- Tire Width
- Aspect Ratio
- Construction Type
- Wheel Diameter
- Load Index
- Speed Index
Let’s assume that you’re going to buy a 315/70R17 tire and you’ve decided to go with Toyo Open Country AT3. When you check to tire’s sidewall, you’ll see,
LT 315/70R17 121/118S
Regarding this size,
LT: Indicates the type of tire
315: Indicates the width of the tire
70: Indicates the aspect ratio of the tire
R: Indicates the construction type of tire
17: Indicates the wheel diameter of the tire
121/118: Indicates the load index of the tire(can be only one number like 121)
S: Indicates the speed index of the tire
Tire Type
LT 315/70R17 121/118S
Tire type is indicated at the beginning of the size. Tires have 4 different types.
- P-Metric: The letter ‘P’ is referring P-metric sizes. P-metric sizes are used for SUVs, CUVs, sedans, coupes, and light-truck owners who want a cozy ride.
- LT: LT means light-truck tire. These tires have stronger sidewalls and generally higher load index.
- No letter: These tires indicate Euro-metric sizes. The load index and inflation pressure of these tires are slightly different than P-metric sizes. However, they’re interchangeable. (P-Metric vs Euro-Metric)
Tire Width
LT 315/70R17 121/118S
The three-digit numbers indicate the width of the tire. The measurement isn’t from edge to edge but from sidewall to sidewall. So, if you’ll measure your tire and see a lower number than the size, no worrise.
Aspect Ratio
LT 315/70R17 121/118S
This element defines sidewall height. But, it is in ratio form. If you want to know the real height of the sidewall, you have to multiply this ratio by tire width (315×0.70=220.5). Though worth reminding, the tire is a glove matter. Therefore, you need a two-section of sidewall if you want to real heigh of the tire.
Construction Type
LT 315/70R17 121/118S
The ‘R’ indicates the construction type of the tires. You can find two different types of construction types in the market.
- R –> Means radial construction, it’s the newest and most efficient building method.
- No letter: If you see a size something like 7.00-16, that means this tire has a cross-ply construction. Even though this construction type is more durable, it heats up so quickly and isn’t usable for any vehicle but tractors and old-school vehicles
Note: Ultra-high performance tires have ZR instead of R. That just means this tire is specific for high-speed usage(above 150mph).
Wheel Diameter
LT 315/70R17 121/118S
These two-digit numbers indicate the wheel diameter. Diameter is the only element that is used in the inch format.
Load Index
LT 315/70R17 121/118S
The load index is one of the most important elements of tire size. It indicates the maximum load that a tire can carry for one axle. These numbers are converted to ‘LBS’ form(you can see the chart above).
You generally see one number instead of two(315/70R17 121S). The second three-digit number indicates the load index for dual usage. Well, the corresponding value of the second number multiplies by two to find load capacity for a dual axle.
Speed Index
This letter indicates the speed capacity of the tire. It’s generally equal to your vehicle’s top-speed capacity.
Since most vehicles don’t reach top speed, some people think that they can use tires with a lower speed index. Well, this isn’t recommended but it’s not dangerous unless you don’t pass over your tire’s speed limit. Though worth reminding, lower speed rating tires mostly have slower steering responses and inferior cornering stability.
Now, you know what these units mean. However, these numbers are meanless if you don’t know your tire size. The next step is finding your tire size.
How to Find My Tire Size?
Finding tire size was way much more difficult early on. However, the usage of the internet has been ramping up in the last 20 years. So, tire manufacturers or online tire dealers develop tons of tire size indication tools. Well, the internet option is always on the table. Yet, I’ll give you a couple of backup plans.
So, I made a deep research about this. You can find too many different tools online. Yet, some of them are really hard you use and ask for too many information.
In my point of view, Michelin’s website is easiest to use.
Michelin Website: https://www.michelinman.com/
Let me briefly explain how to use it,
- Open Michelin’s webpage
- Find ‘Find your tire in Second’ section
- Select your vehicle type on the left
- Click ‘Select your Year’ or you can simply click the white search box at the middle
- Select your vehicles’ production year
- Select your vehicles’ manufacturer
- Select your model
- Select your trim level
- Sizes will be listed on the screen
- You might see a couple of sizes, you’re going to go with your wheel size(simply check the current tires and find the two digit number come after ‘R’)
Owner's Manuel
The other method for a finding your tire size is using owner’s manuel. If you’re the first owner of the vehicle, this book is delivered by the manufacturer of your vehicle. However, subsequent owners generally doesn’t get the hard copy of this book.
The size information can be found under the tire or wheel information.
Online owner’s manuals are reachable on manufacturer’s websites. I’m sharing the links for popular vehicle manufacturers below.
- FORD –> https://www.ford.com/support/owner-manuals/
- SUBARU –> https://www.subaru.com/owners/vehicle-resources.html
- TESLA –> https://www.tesla.com/ownersmanual
- CHEVROLET –> https://www.chevrolet.com/support/vehicle/manuals-guides
- TOYOTA –> https://www.toyota.com/owners/resources/warranty-owners-manuals
- HONDA –> https://owners.honda.com/vehicle-information/manuals
- GMC –> https://www.gmc.com/support/vehicle/manuals-guides
- DODGE, CHRYSLER, FIAT, JEEP, RAM, WAGONEER –> https://www.mopar.com/en-us/my-garage/select-vehicle.html
Note: Trim levels of vehicles can be changed from country to country. Hence, you better use country based websites of the manufacturers.
Driver's Door Jamb
The tire size can be found on your driver’s door also. When you open your door, you may find it on two places.
- Door jamb
- The curve that door sits
Glove Box Door
A bit rare but you can also check your glove box. You’ll look for the above sticker.
Gas Tank Hatch Door
Above sticker can be found on the gas tank hatch door also. However, when the ink encounter with the gas, it wipe away. So, most of these stickers become useless in time.
Conclusion
I hope the article was helpful. The selection is crucial for efficient performance. Moreover, for a long-lasting rubber.
If you’d like to know how to make your tires last longer –> https://tireterrain.com/how-to-make-tires-last-longer/
Take-Home Points,
- LT sizes have stronger sidewalls. That feature makes them more durable but less cozy. If you’re going to use them only for city driving. I don’t recommend them.
- XL(vs SL) sizes tend to carry more load. Hence, they might be a better pick for hauling and towing
- Please pay attention to load range and speed index. These metrics are highly important if you expect decent tread life.
I hope the article was helpful. If you have any further questions, please leave them into the below section.
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