The side of your tires can have a lot of information. There’s not only your tire size, but also the speed rating, load rating, recommended inflation, DOT...the list goes on! Most people can pick out the size, but everything else can end up looking like a random jumble of numbers and letters. But understanding these numbers can be crucial when you are buying tires, especially if your vehicle has special requirements. For example, if you have a light truck that carries heavy loads, then knowing what to look for can impact the safety of your vehicle.
Breaking It Down: Speed Ratings
The tire speed rating is indicated by a letter found on the sidewall of your tire. Each letter represents a speed (shown in both km/h and mph) that is the maximum the manufacturer says the tire can sustain for long periods of time, at least without falling apart. Now this doesn’t necessarily mean that that’s the maximum speed you can drive (although they’re usually way higher than normal driving speeds outside of the Autobahn) it’s just the maximum that the manufacturer expects the tire to be able to maintain for a long time. In other words, if a tire is rated “S”, then the tire will perform for an extended period of time at 112 mph and, although it won’t break apart the instant you hit 113 mph, it’s not recommended to sustain speeds higher than 112 mph.
The official tire speed rating system in the United States begins at A1 (a maximum MPH of 3) and ends at Y (186 MPH). As these things go, tires and vehicles have evolved since the inception of this speed rating system, so additional ratings have been added as needed. This is why Z and Y have such a big increase from the rest of the letters.
So how can you tell what the letter means? Well in km/h it’s pretty easy, but since we use mph there’s not really a simple way. You’ll just have to look at a chart like the one below:
You can usually find these charts online pretty easily, but if you don’t have access to the internet when you need to know, you can always match to your current tires, check what’s listed in the door of your vehicle, or ask a mechanic. However, as most speed ratings are so high, it’s rare you’ll have an issue with them.
The load index is a number, usually found next to the speed rating, that represents the maximum load that the tire can carry. However, much like the letters of the speed rating, the numbers are a code rather than the actual pounds. For example, a tire that has a load index of 100 will be able to carry a maximum of 1,764 lbs (nearly a ton). The load index codes run from 0 (99 pounds) to 150 (7,385 pounds). But don’t forget, your vehicle has 4 tires, and it’s usually carrying passengers and items that can all add to the weight. So if you’re looking for a load rating that works for you, you can multiply the pounds by 4 and you’ll get the total amount the tires can hold together. Just remember to add a bit extra from your vehicle weight to account for people and things you might have in your car! Or play it safe and match to what’s on your vehicle now (or what’s listed in the door).
Those are the basics of load ratings, but unlike speed ratings, the load ratings can get a little more complicated. For example, if you have a truck you may need to carry more than just the usual loads of a vehicle.
Passenger vs. LT Tires
Passenger (or P) tires are meant primarily for passenger vehicles, like cars and SUVs (although there are also some trucks that take P tires). P tires can be marked with a with a “P” before the tire size, but this isn’t always the case. Just as often, the size of P tires will have nothing in front of it. However, passenger tires will have the basic kind of load index we discussed above.
Light Truck (or LT) tires are usually marked with an LT before the size. This shows right away that the tire is intended for “Light truck-metric” vehicles. Basically this is just a vehicle that carries a heavy cargo load, like a moving van, some pickup trucks, or vehicles designed to pull a trailer. This does not, however, include things like semi trucks and 18 wheelers, which are a whole different category all together. LT load ratings also look different. They’ll usually have 2 load index numbers separated by a slash, designating the full load capacity of the tires. Some, though, will just have a letter (usually C, D or E) that indicates the tire ply rating. Here’s an example chart for the letter ratings:
This can get confusing, so it’s usually best to stick with what’s recommended for your vehicle, or consult a mechanic if you’re looking to change. It’s also worth noting that you don’t want to mix LT and P tires.Because they’re designed to carry more weight, the sidewalls of LT tires are usually thicker and not as flexible as passenger tires, so you wouldn’t want them both on your vehicle at the same time.