What Do Tire Numbers Mean?
Have you ever wondered what all those numbers on your tires really mean? Watch the video below and in less than 2 minutes you'll be an expert on tire numbers!
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The graphic to the right has four numbers and two letters, and all of them are important. Here's each number and letter in order, as well as what it represents. A more detailed explanation of each characteristic is below.
- 255 - This is the tire's width in millimeters
- 45 - This is the tire's sidewall height, expressed as a percentage of the width figure, also known as an aspect ratio
- R - This is the tire's type of construction, and is almost always an "R"
- 15 - This is the wheel diameter the tire is designed for, listed in inches
- 99 - This is the load rating
- Y - This is the tire's speed rating
More Detail On Specific Tire Characteristics
Generally speaking, wider tires provide better handling and traction at high speed, at least on dry pavement. This is why most sports cars have wider-than-average tires - when taking a corner at high speed, a tire's tendency is to "roll-over" (meaning the sidewall flexes). By making the tires wider, the negative effects of sidewall flexing are reduced.
However, while wider tires are great for racing, they're not too good for fuel economy. Wider tires have more rolling resistance, which means your engine has to work just a little bit harder to move your car. Therefore, most economy cars and almost all hybrids have narrower-than-average tires.
So, to sum up:
- Wider is better for handling and performance (racing)
- Narrower is better for efficiency (better gas mileage)
NOTE: Some off-road tires are described using English units (meaning inches instead of millimeters). These tires are usually described by width and total height. For example, a 10.5″ x 33″ tire is 10.5″ wide and 33″ in total height.
The sidewall height is described as a percentage of the tire width. In our example, our tires are 225mm wide. Since the sidewall height is 45% of the width, the sidewall is 101.25mm tall (approximately). As you can see, using a percentage to describe sidewall height makes the numbers a little cleaner.
As a general rule, taller sidewalls result in a smoother ride. If you're buying tires for your 1984 Lincoln Continental, for example, the sidewall height is a very generous 75 series (the OE tire size is 205/75R15). If you're buying tires for a brand-new sports car, you might find that the sidewall height is a very slim 35 series (newer BMW M3′s can wear tires as low profile as 265/35R19). These are called "low profile" tires, and they're usually found on sports cars because a) low profile tires make for a better handling car and b) sports car owners aren't likely to complain about a stiff ride.
- Taller (high aspect ratio) sidewalls usually make for a smoother, more luxurious ride.
- Low profile tires (low aspect ratio) have short, skinny sidewalls, which are great for maximizing a vehicle's cornering ability. The downside is that a vehicle with low profile tires usually has a stiff ride-there's no sidewall to absorb bumps.
Tire Construction Types
The vast majority of tires in the modern world are radial tires (R), simply because radial construction provides the best combination of performance and durability. Before the advent of the radial tire (it was invented in the early 1900′s, but it wasn't commercially viable until the 1950′s - see wikipedia), most vehicles wore bias-ply tires.
While bias-ply tires are very flexible (a characteristic that makes them useful to off-road enthusiasts to this day), they're no longer used on passenger vehicles.
Note: If you'd like to learn more about how bias-ply tires are used in off-roading, read this article on TacomaHQ.com.
Over the last 20 or so years, automakers have been equipping new vehicles with progressively larger wheels. Take the Toyota 4Runner, for example. Here are the largest wheels that Toyota has offered as original equipment since 1995:
- 1995 4Runner - 15″
- 2001 4Runner - 16″
- 2006 4Runner - 18″
- 2012 4Runner - 20″ is the largest size, and you can't go smaller than 17″
As you can see, wheel diameters have grown substantially in the last 20 years or so on the 4Runner, and it's also true for most cars on the road. The primary reason wheel diameters have increased is cosmetic - consumers prefer the look of larger wheels. However, the increased use of disc brake systems - and the increased emphasis on brake performance - has also forced wheel diameters to increase in order to accommodate larger rotors and calipers.
Still, the performance benefits of larger wheels are debatable. While larger wheels have a greater rolling mass - and are therefore less likely to be upset by bumps in the road (better ride), they're also heavier. Heavier wheels place more of a strain on the suspension system (greater unsprung weight), so larger wheels don't usually improve a vehicle's ride substantially.
Also, because larger wheels are heavier than smaller wheels, they can negatively impact acceleration. Many a drag racer has removed expensive 18″ or 20″ wheels with cheaper (and lighter) 15″ wheels in order to improve their time in the quarter mile.
- Larger diameter wheels tend to look better, but they don't necessarily improve ride or performance in and of themselves (only larger wheels often have low-profile tires, and low-profile tires DO improve handling).
Load Rating (aka Load Index)
Unless you own a pickup or commercial truck, you're not likely to care too much about a tire's load rating. Ratings usually fall between 70 and 110, with higher ratings for stronger tires. Tires with a high load rating are great for hauling the mail, but they're expensive to replace, they're usually not as fuel efficient, and they don't have as much "flex" so they tend to ride more roughly than tires with a lower rating.
If you're a truck or SUV owner shopping for tires, it's a good idea to assess just how much hauling and towing you do before you lay down big money for a set of heavy-duty tires. Most truck and SUV owners are perfectly satisfied with OE replacement tires rather than heavy-duty "load range E" tires, which have a higher-than-average load rating (the exact rating depends on the size of the tire).
If you've ever heard someone talk about "Z-rated" tires, you understand the exuberance that a tire's speed rating can inspire. While most vehicles have tires with speed ratings of R, S, or T, sports cars can wear tires rated V, W, Y, or Z.
So what do these ratings mean? Maximum sustainable speed. Z-rated tires are built to withstand extremely high temperatures, so they can be used at speeds over 149mph. R-rated tires, on the other hand, aren't as temperature resistant, so they're only good up to speeds of 106mph. Clearly, most tires are rated for speeds that exceed all but the most extreme daily commute. Therefore, most consumers don't need to pay too much attention to tire speed ratings.
Still, just for reference, here's a list of all speed ratings and corresponding sustained speed limits:
- L - 75mph —
- M - 81mph
- N - 87mph
- P - 93mph
- Q - 99mph
- R - 106mph —
- S - 112mph
- T - 118mph
- U - 124mph
- H - 130mph —
- V - 149mph
- W - 168mph
- Y - 186mph
All efforts were made to develop an accurate resource. However, nobody's perfect. Therefore, while we're very confident, we take no responsibility for the total and complete accuracy of our work-don't say we didn't warn you.
If you find an error, please contact jason [at] tundraheadquarters.com so we can get it fixed.