If you look at your tires you’ll find all kinds of different numbers, letters, symbols… maybe even hieroglyphics. To the untrained eye these might seem like some type of secret code, but we promise it isn’t. This is the vital info you need to match the correct tires to your wheels. By the end of this article, you’ll know everything you need to confidently buy tires that 1) fit your wheels and 2) perform the way you want.

Tire specs don’t mean a lot by themselves. Understanding the wheels they go on adds a whole lot more context. Luckily, we already took a deep dive into all the important wheel specs you should know. If you haven’t seen it, be sure to check it out.

Everything you need to know about wheel specs

If you’re buying tires for your truck, we got you covered. This guide is made for both car & truck owners. First, we'll cover the metric sizing system, the one you’ll typically find, and later, flotation - truck owners should know both.

Quick links

• Tire width (metric)
• Aspect ratio
• Tire construction
• Wheel diameter
• Load index
• Tire diameter
• Tire width (flotation)
• Treadwear
• Traction
• Temperature

Metric sizing (the norm)

Probably the first thing you’ll see on a tire, apart from make and model, is a string of numbers that looks something like this: 255/40R17. These numbers tell you the physical dimensions of a given tire.

Tire width

The first number we see is three digits long (255/40R17). This denotes how wide the tires are. This is almost always expressed in millimeters, if you suspect it's inches, you’re probably dealing with flotation, which we’ll talk about later. In our example, a 255 tire is 255mm wide, or 10”.

What is the perfect size for your rim? You’re thinking a 255 tire and 10” wide wheel are a match made in heaven, right? Not according to tire manufacturers. Most say the ideal tire is one size larger than the width of the wheel. This is because our 10” wheels will actually measure closer to 11” if we include the bead seats.

So, for our 10” wide wheels, the ideal fit would be 265 or 275 wide tires (according to manufacturers). This will give you a perfectly square sidewall - neither stretched nor ballooned.

Of course, you can play around with sizing a bit to achieve a ‘stretch’, or the opposite, oversized tires that look like diapers on your wheels. Stretched tires are a topic for a different day, luckily we already made a YouTube video on them:

Tire aspect ratio

Of all the numbers relating to sizing, aspect ratio definitely gets confused the most. This is the two-digit number that follows the tire width (255/40R17). This number will dictate how tall the sidewall of your tires is.

The reason it causes so much confusion is because this number is a percentage of the width rather than an inch or millimeter measurement. In our example of a 255/40, the sidewall will measure 102mm, as 40% of 255mm = 102mm.

What you need to remember when comparing tires is that the sidewall will change if you go to a wider or narrower tire, despite the number staying the same. A 235/40 will have a smaller sidewall than a 255/40.

Besides fitment, what does sidewall do? Generally, a smaller sidewall will handle better as there's less material to flex, while a larger sidewall will give a more comfortable ride.

Tire construction

There will be a letter or two following the aspect ratio (255/40R17). You’ll see ‘R’ most of the time. All this means is that the ply of the tire follows a radial pattern. You don’t really need to pay attention to this, as it’s pretty default on DOT-compliant tires. If it reads ‘D’ or ‘B’ instead, you’re probably looking at a tire for heavy machinery or trailers.

For performance tires, you might see ‘ZR’, which indicates a higher speed rating. This would read 255/40ZR17.

Wheel diameter

The two-digit number that follows tire construction is the wheel (rim) diameter (255/40R17). If you get this one wrong, forget the rest, because you won’t even be able to mount the tires on your wheels.

Luckily, it’s pretty self-explanatory, just match the tire size to the wheel diameter.

A 255/40R17 will only work on a 17” diameter wheel.

Load Index

Everything so far relates to sizing. After the main string, there’s normally a space followed by a few additional characters (255/40R17 94W). These have nothing to do with sizing, but instead performance.

First is a two or three-digit number. This is the load index or ‘load rating’. Realistically, if you’re buying tires for your car, you don’t need to pay too much attention to this. But for truck owners, the general idea is: the higher the number, the more weight the tires can hold.

If you don’t like math, you can look up the capacity in pounds based on this number. If we have a 116 load index, this translates to 2756 lbs. Multiply this by the number of tires, and you have the total carrying capacity of the vehicle. 11,024 lbs in this case.

If there are two numbers, such as 120/116. The first number, 120, is for single tires, while 116 is for a dually setup.

Speed rating

The final character that may be present is a speed rating (255/40R17 94W). This one is pretty simple, your tires are rated up to X speed.

Speed Rating
‎Value	Max speed
V	Up to 149 mph
W	Up to 168 mph
Y	Up to 186 mph

Flotation sizing (offroad vehicles)

Flotation tires aren’t built differently, they’re just measured differently. You can simply consider them imperial-sized tires - as all measurements are in inches. Your BFG K02s are going to perform exactly the same whether they use metric or flotation. If your looking for tall AND wide tires, you’re perfect size might only be possible in flotation.

Here’s an example of a tire in flotation terms: 33x10.5-15.

Tire diameter (rolling diameter)

The big change from the metric system is that the first number (33x10.5-15) relates to the sidewall and the second to width, opposite of metric.

The first number is an absolute measurement in inches of the overall rolling diameter. In our example, the tire would be 33” inches, regardless of the other numbers. This system focuses on tire height, which makes a lot of sense for trucks.

Tire width

After diameter, the next measurement is for width (33x10.5-15); also an absolute measurement expressed in inches. In our example, this would be a 10.5-inch wide tire. Again, flotation tires generally accommodate wider sizes. If you want skinnies (pizza cutters), you’re more likely to find them in metric sizing.

The unwritten rules of ‘what looks good’ on a truck are completely different than on a car. You should absolutely get tires that are wider than your wheels… by a few sizes. For our example tire, an 8” or 8.5” wide rim would look great.

And those are the only differences between flotation and metric

Tire construction, wheel diameter, load index, and speed rating all work the same flotation as in metric (33x10.5R15120/116V).

UTQG (Performance numbers)

If you’ve made it this far, congrats, you know everything you need to know about tire sizing. You can confidently order tires for your show build, daily, etc.

If you plan to use your car at the track, there are a few more numbers you should know about. You can find treadwear, traction, and temperature info in a different spot on the tire. This is called UTQG (Uniform tire quality grading). These ratings give an at-a-glance idea of how well a tire performs.

Treadwear, traction, and temperature ratings can be found in this order and all of them will be spelled out in plain English.

Treadwear

This number rating tells how hard’ a tire is. The lower the treadwear, the softer and grippier a tire is. A higher treadwear won’t have as much grip, but the tradeoff is it will last longer.

Typical values range between 200-600.

Treadwear
‎Value	Recommended use
<200	Competitive track
200-300	Occasional track + street
300-400	Weekend street car
400+	Daily driver

Traction

Traction grades go from C to A, and sometimes up to AA. They estimate how well a tire will stop in wet conditions; C being the worst, AA being the best. Most passenger car tires these days are A-rated.

Temperature

Temperature grades also go from C to A. This rating is all about how a tire can handle continued heat without prematurely failing. C is the minimum for DOT-approved tires. B and A grades will often be found on high-performance tires.

Wrap up

It’s amazing how many people can rebuild an engine or transmission, but still don’t understand aspect ratio - don’t be that guy. If you paid attention, you now know more about tires than the majority of car enthusiasts.

Hopefully, you also know about wheel sizing. But if not, no worries, we also made a guide all about that.

When you’re ready to pick out your dream set of wheels, head over to our shop to find something for your car or truck. The best part, you can get wheels & tires bundled and shipped directly to your door.