Cueball
01-04-2009, 03:25 PM
How to read the sidewall of your tire
and just what the heck is UTQGS?
Historically the springtime is when buds form on bare trees, grass grows tall in fertile fields, and the gentle breezes bring with them the smell of new tires. That's right, new tires. Springtime is tire season when car freaks of all types prep their cars for the upcoming summer of events and trips. Springtime is also the time when tire manufacturers price their tires at the highest cost while the best deals are actually in the Winter, January is usually the best.
There is a lot of information on a tire's sidewall. Hopefully this article will shed some light on some of it. You are probably up to speed on what the various numbers in a tire’s size mean, but just in case you need a quick refresher, lets take a tire sized 215/55ZR16 93W. 215 is the tire’s section width (the width of the tire tread sidewall-to-sidewall) in millimeters. The 55 is the tire’s aspect ratio, or the ratio of its sidewall height to the tread width (in this case, the sidewall height is 55 percent of the tire’s width). The Z in ZR indicates the tire is speed rated to exceed 149 mph (though the actual speed rating appears with the load rating after the size designation). The R means the tire is of radial construction. The 16 is the tire’s rim diameter. The 93 is the tire’s load index, an arbitrary number that corresponds to how much weight the tire can carry. In this case, the 93 means the tire is rated to carry 1,433 pounds. The W is the tire’s speed rating, a measure of the maximum speed at which the tire can carry its rated load. This tire is speed rated up to 168 mph. ( You’ll find more information on speed ratings later in the article)
Now lets turn to some fun stuff called the Uniform Tire Quality Grading System, or UTQGS. These are Government ratings that evaluate three aspects of a tire’s performance capabilities. They are:
Tread ware -A numeric rating that describes the life, or wear rate, of a tire’s tread based on controlled laboratory testing conditions. You could also consider this a measure of how soft or sticky a tire’s tread compound is. The lower the number, the stickier the tire. A tire with a tread wear rating of 300 will wear twice as long as one with a 150, but I’d bet the 150 tire would be more fun while it lasted.
Traction - A letter grade (AA,A,B,C with AA being best) that rates the tire’s traction capability during a low-speed (40 mph) straight-line braking test on wet pavement. Note that the test is of straight-line braking; this grade does not address a tire’s cornering ability. Also note that the AA grade has been in place since 1997 and applies only to tires that have been introduced since then. An A grade on an older line is the best it can achieve.
Temperature - Another letter grade system (A, B, C with A being the best) that represents the tire’s resistance to the generation of and its ability to dissipate heat. Heat is a tire’s number-one enemy, as heat can cause a tire to fail catastrophically. All passenger tires sold in the US must have at least a C rating.
I have included the UTQGS ratings as I feel they are a good indicator of a tire’s performance capabilities.
A tire’s speed rating is a letter-based designation, printed on the tire’s sidewall in conjunction with its load rating, to indicate the maximum speed at which the tire can carry the load under test conditions.
In our 215/55ZR16 93W example the letter W at the end is the speed rating. As you’ll see from the chart below, W means the tire is good for up to 168 mph or 270 kph.
Speed Ratings
Rating MPH KPH
H 130 210
V 149 240
W 168 270
Y 186 300
ZR >149 >240
The fact that the mph is a weird number and the kph is not should tip you off that the whole rating system began in metric-crazy Europe where the ratings step in even kph increments. For those of you who might have their hands on a comprehensive speed rating chart will see that ratings go all the way from A1 (3 mph) to Z (over 149 mph). The chart covers only those ratings that pertain to high and ultra-high performance tires.
Plus sizing is a buzz word in the tire industry. The concept behind plus sizing tires and wheels is to increase our rim diameter an inch or more compared with stock while keeping the tire’s overall diameter the same. In other words, as the wheel gets bigger, the sidewall gets shorter, but the overall height doesn’t change. This will get you the bling-bling rims you want without completely throwing off your engine computer or needing to swap gear ratios to compensate for taller tires.
In plus sizing terminology, Plus 1 means gaining an inch in wheel diameter, and Plus 2 means going up 2 inches. Some tire and wheel retailers, including Discount Tire, also have a Plus 0 category. thats a recommendation for going up in tire size (to maximize the amount of rubber on the road) without a corresponding increase in wheel diameter. Discount Tire (www.tires.com) and the Tire Rack (www.tirerack.com) are two sites you can go to check out tire size recommendations for plus sizing for your Beetle.
Plus sizing has benefits beyond the aesthetic ones, too. Generally, when you shorten a tire’s sidewall, it gets stiffer. That can translate to a harsher ride, but it will also make dramatic improvements to your car’s handling, as the shorter, stiffer sidewalls won’t deflect, or roll over, as much during extreme handling maneuvers. Steering will immediately feel crisper, as your steering input will have much more direct result. The downside to a shorter sidewall is the possibility of damaging your wheels when hitting road hazards like pot holes or railroad tracks.
Anytime you swap tires, whether you are plus sizing or not, be sure the new tires have the same (or better) load and speed ratings as your original equipment. Those ratings are a measure of the tire’s strength - its ability to carry a load, disperse heat through the tread and carcass, and so on. The last thing you want to do, especially if you’re buying new tires to drive your car harder, is heap more punishment on the tires than they’re built to handle. Overloading tires is not safe.
You may find, however, that to maintain the tire’s load rating as the sidewall gets shorter you have to go with a wider tire. That’s because the load rating is based on the volume of air in the tire, and if one dimension of the tire is getting smaller, another may have to grow to maintain that volume. In many cases there is room in the wheel wells to accommodate the extra width (which is why the Plus 0 concept works), but make sure you have adequate clearance to fit the wider tires, even at the extremes of your turning radius.
Well that is the first episode of Cueball in the Side Pocket. Hope you enjoyed it. Remember to keep the shiny side up and have safe fun out there!
and just what the heck is UTQGS?
Historically the springtime is when buds form on bare trees, grass grows tall in fertile fields, and the gentle breezes bring with them the smell of new tires. That's right, new tires. Springtime is tire season when car freaks of all types prep their cars for the upcoming summer of events and trips. Springtime is also the time when tire manufacturers price their tires at the highest cost while the best deals are actually in the Winter, January is usually the best.
There is a lot of information on a tire's sidewall. Hopefully this article will shed some light on some of it. You are probably up to speed on what the various numbers in a tire’s size mean, but just in case you need a quick refresher, lets take a tire sized 215/55ZR16 93W. 215 is the tire’s section width (the width of the tire tread sidewall-to-sidewall) in millimeters. The 55 is the tire’s aspect ratio, or the ratio of its sidewall height to the tread width (in this case, the sidewall height is 55 percent of the tire’s width). The Z in ZR indicates the tire is speed rated to exceed 149 mph (though the actual speed rating appears with the load rating after the size designation). The R means the tire is of radial construction. The 16 is the tire’s rim diameter. The 93 is the tire’s load index, an arbitrary number that corresponds to how much weight the tire can carry. In this case, the 93 means the tire is rated to carry 1,433 pounds. The W is the tire’s speed rating, a measure of the maximum speed at which the tire can carry its rated load. This tire is speed rated up to 168 mph. ( You’ll find more information on speed ratings later in the article)
Now lets turn to some fun stuff called the Uniform Tire Quality Grading System, or UTQGS. These are Government ratings that evaluate three aspects of a tire’s performance capabilities. They are:
Tread ware -A numeric rating that describes the life, or wear rate, of a tire’s tread based on controlled laboratory testing conditions. You could also consider this a measure of how soft or sticky a tire’s tread compound is. The lower the number, the stickier the tire. A tire with a tread wear rating of 300 will wear twice as long as one with a 150, but I’d bet the 150 tire would be more fun while it lasted.
Traction - A letter grade (AA,A,B,C with AA being best) that rates the tire’s traction capability during a low-speed (40 mph) straight-line braking test on wet pavement. Note that the test is of straight-line braking; this grade does not address a tire’s cornering ability. Also note that the AA grade has been in place since 1997 and applies only to tires that have been introduced since then. An A grade on an older line is the best it can achieve.
Temperature - Another letter grade system (A, B, C with A being the best) that represents the tire’s resistance to the generation of and its ability to dissipate heat. Heat is a tire’s number-one enemy, as heat can cause a tire to fail catastrophically. All passenger tires sold in the US must have at least a C rating.
I have included the UTQGS ratings as I feel they are a good indicator of a tire’s performance capabilities.
A tire’s speed rating is a letter-based designation, printed on the tire’s sidewall in conjunction with its load rating, to indicate the maximum speed at which the tire can carry the load under test conditions.
In our 215/55ZR16 93W example the letter W at the end is the speed rating. As you’ll see from the chart below, W means the tire is good for up to 168 mph or 270 kph.
Speed Ratings
Rating MPH KPH
H 130 210
V 149 240
W 168 270
Y 186 300
ZR >149 >240
The fact that the mph is a weird number and the kph is not should tip you off that the whole rating system began in metric-crazy Europe where the ratings step in even kph increments. For those of you who might have their hands on a comprehensive speed rating chart will see that ratings go all the way from A1 (3 mph) to Z (over 149 mph). The chart covers only those ratings that pertain to high and ultra-high performance tires.
Plus sizing is a buzz word in the tire industry. The concept behind plus sizing tires and wheels is to increase our rim diameter an inch or more compared with stock while keeping the tire’s overall diameter the same. In other words, as the wheel gets bigger, the sidewall gets shorter, but the overall height doesn’t change. This will get you the bling-bling rims you want without completely throwing off your engine computer or needing to swap gear ratios to compensate for taller tires.
In plus sizing terminology, Plus 1 means gaining an inch in wheel diameter, and Plus 2 means going up 2 inches. Some tire and wheel retailers, including Discount Tire, also have a Plus 0 category. thats a recommendation for going up in tire size (to maximize the amount of rubber on the road) without a corresponding increase in wheel diameter. Discount Tire (www.tires.com) and the Tire Rack (www.tirerack.com) are two sites you can go to check out tire size recommendations for plus sizing for your Beetle.
Plus sizing has benefits beyond the aesthetic ones, too. Generally, when you shorten a tire’s sidewall, it gets stiffer. That can translate to a harsher ride, but it will also make dramatic improvements to your car’s handling, as the shorter, stiffer sidewalls won’t deflect, or roll over, as much during extreme handling maneuvers. Steering will immediately feel crisper, as your steering input will have much more direct result. The downside to a shorter sidewall is the possibility of damaging your wheels when hitting road hazards like pot holes or railroad tracks.
Anytime you swap tires, whether you are plus sizing or not, be sure the new tires have the same (or better) load and speed ratings as your original equipment. Those ratings are a measure of the tire’s strength - its ability to carry a load, disperse heat through the tread and carcass, and so on. The last thing you want to do, especially if you’re buying new tires to drive your car harder, is heap more punishment on the tires than they’re built to handle. Overloading tires is not safe.
You may find, however, that to maintain the tire’s load rating as the sidewall gets shorter you have to go with a wider tire. That’s because the load rating is based on the volume of air in the tire, and if one dimension of the tire is getting smaller, another may have to grow to maintain that volume. In many cases there is room in the wheel wells to accommodate the extra width (which is why the Plus 0 concept works), but make sure you have adequate clearance to fit the wider tires, even at the extremes of your turning radius.
Well that is the first episode of Cueball in the Side Pocket. Hope you enjoyed it. Remember to keep the shiny side up and have safe fun out there!