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#3171 - 10/19/09 09:09 AM How to Read Speed Rating, Load Index, Service Descriptions  
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Using a P195/60R15 87S tire size as an example, the 87S at the end of the size represents the tire's service description. A service description identifies the tire's load index and speed rating. Service Descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991.

The first two digits (87S) represent the tire's load index and are followed by a single letter (87S) identifying the tire's speed rating.

Load Index

P195/60R15 87S - The load index (87) is the tire size's assigned numerical value used to compare relative load carrying capabilities. In the case of our example the 87 identifies the tires ability to carry approximately 1,201 pounds.

The higher the tire's load index number, the greater its load carrying capacity.

89 = 1,279 pounds
88 = 1,235 pounds
87 = 1,201 pounds
86 = 1,168 pounds
85 = 1,135 pounds

A tire with a higher load index than that of the Original Equipment tire indicates an increase in load capacity. A tire with a load index equal to that of the Original Equipment tire indicates an equivalent load capacity. A tire with a lower load index than the Original Equipment tire indicates the tire does not equal the load capacity of the original.

Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 110.

Load Index Pounds Kilograms

71 761 345 91 1356 615
72 783 355 92 1389 630
73 805 365 93 1433 650
74 827 375 94 1477 670
75 853 387 95 1521 690
76 882 400 96 1565 710
77 908 412 97 1609 730
78 937 425 98 1653 750
79 963 437 99 1709 775
80 992 450 100 1764 800
81 1019 462 101 1819 825
82 1047 475 102 1874 850
83 1074 487 103 1929 875
84 1102 500 104 1984 900
85 1135 515 105 2039 925
86 1168 530 106 2094 950
87 1201 545 107 2149 975
88 1235 560 108 2205 1000
89 1279 580 109 2271 1030
90 1323 600 110 2337 1060


Speed Rating

In Germany some highways do not have speed limits and high speed driving is permitted. Speed ratings were established to match the speed capability of tires with the top speed capability of the vehicles to which they are applied.
Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at "unusual" mile per hour increments).
Despite the tire manufacturer's ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits. The maximum operating speed of a vehicle must be limited to the lowest speed rated tire on the vehicle.

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.

It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer's original speed rating, even after being repaired because the tire manufacturer can't control the quality of the repair.

Over the years, tire speed rating symbols have been marked on tires in any of three ways shown in the following examples:

225/50SR16 225/50SR16 89S or 225/50R16 89S


Each of these was an acceptable method of identifying speed ratings.

Early tires had their speed rating symbol shown "within" the tire size, such as 225/50SR16. Tires using this type of branding were not to have been produced after 1991.

225/50SR16 112 mph, 180 km/h
225/50HR16 130, 210 km/h
225/50VR16 in excess of 130 mph, 210 km/h


Beginning in 1991, the speed symbol denoting a fixed maximum speed capability of new tires must be shown only in the speed rating portion of the tire's service description, such as 225/50R16 89S.
The most common tire speed rating symbols, maximum speeds and typical applications are shown below:

L 75 mph 120 km/h Off-Road & Light Truck Tires
M 81 mph 130 km/h
N 87 mph 140km/h Temporary Spare Tires
P 93 mph 150 km/h
Q 99 mph 160 km/h Studless & Studdable Winter Tires
R 106 mph 170 km/h H.D. Light Truck Tires
S 112 mph 180 km/h Family Sedans & Vans
T 118 mph 190 km/h Family Sedans & Vans
U 124 mph 200 km/h
H 130 mph 210 km/h Sport Sedans & Coupes
V 149 mph 240 km/h Sport Sedans, Coupes & Sports Cars


When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph.
While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of new vehicles that have extremely high top-speed capabilities.

W 168 mph 270 km/h Exotic Sports Cars
Y 186 mph 300 km/h Exotic Sports Cars


While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire's 168 mph, 270 km/h maximum speed.

225/50ZR16 in excess of 149 mph, 240 km/h
205/45ZR17 88W 168 mph, 270 km/h
285/35ZR19 99Y 186 mph, 300 km/h


Most recently, when the Y-speed rating indicated in a service description is enclosed in parentheses, such as 285/35ZR19 (99Y), the top speed of the tire has been tested in excess of 186 mph, 300 km/h indicated by the service description as shown below:

285/35ZR19 99Y 186 mph, 300 km/h
285/35ZR19 (99Y) in excess of 186 mph, 300 km/h


As vehicles have increased their top speeds into Autobahn-only ranges, the tire speed ratings have evolved to better identify the tires capability, allowing drivers to match the speed of their tires with the top speed of their vehicle.


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#3172 - 10/19/09 09:15 AM Re: How to Read Speed Rating, Load Index, Service Descriptions [Re: teamzr1]  
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Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles.

Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire's structure, blending the tire's responsiveness, traction and handling.

Disadvantages of Underinflation

An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle's tires are underinflated by only 6 psi it could weaken the tire's internal structure and eventually lead to tire failure.
Lower inflation pressures will allow more deflection as the tire rolls. This will build up more internal heat, increase rolling resistance (causing a reduction in fuel economy of up to 5%) and reduce the tire's tread life by as much as 25% while increasing the probability of irregular treadwear.
Drivers would also find a noteworthy loss of steering precision and cornering stability. While 6 psi doesn't seem excessively low, it typically represents about 20% of a passenger car tire's recommended pressure.

Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear.
Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior.
However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures.


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#3173 - 10/19/09 09:17 AM Re: How to Read Speed Rating, Load Index, Service Descriptions [Re: teamzr1]  
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Air Pressure vs. Dry Performance

Most drivers realize that tire load capacity is determined by tire size and inflation pressure. Larger tires and higher inflation pressures provide more load capacity, while smaller tires and lower tire pressures provide less.

Correctly inflated tires receive appropriate support from the contained air pressure to provide an even distribution of load across the footprint and help stabilize the tire's structure. And while most drivers recognize that this has a significant impact on tire wear, rolling resistance and durability, only a few realize underinflation also has a noticeable influence on how quickly and precisely the tires respond to the driver's input.

In order to evaluate the influence of inflation pressure on response and handling, the Tire Rack conducted a Performance Test Track Drive, comparing properly inflated tires to purposely underinflated tires. We used 2003 BMW 330Ci coupes, and installed P225/50R16 tires on 16x7.5" wheels. We tested new, full tread depth tires.

We chose Goodyear Eagle GT-HR High Performance All-Season radials that were developed to blend good treadwear, responsive handling and dependable traction. One of the Eagle GT-HR's highlights is its internal construction that features Goodyear's RaceWrap Construction Technology developed for the Eagle Race tires used in NASCAR competition.

RaceWrap Construction Technology brings a casing ply down the sidewall at a slight angle, wraps around the bead and returns it all of the way up the sidewall until it ends under the edge of the steel belts. This slightly angled, two-ply sidewall enhances steering response and handling stability.

The tires installed on one of our BMW 330Ci test cars were inflated to the vehicle manufacturer's recommended inflation pressures of 29 psi front and 33 psi rear, while the other car had its tires inflated 30% lower (20 psi in the front and 23 psi in the rear).
We chose 30% underinflation because it was the percentage of loss initially established by the US DOT at which passive pressure monitoring systems must warn the driver of low inflation pressure on future cars.

The first part of the test was visual. We asked the drivers to look at the tires and decide which of the two vehicles was equipped with the underinflated tires. While perhaps this visual test might have been easier with taller tires of the past, today's low profile tires fitted to the BMWs demonstrated how difficult it has become.
The drivers agreed that the tire appearance alone did not provide irrefutable confirmation of the tire pressure contained inside. You can't use your eyes as a tire pressure gauge.

The next test was run on our test course to confirm the influence of tire pressure on the tire's performance at its limit. While the drivers thought that the properly inflated tires provided responsiveness and predictable handling, they quickly realized that the same tires in an underinflated state left a lot to be desired. The underinflated tires required more steering input to initiate maneuvers and were slower to respond. The underinflated tires also felt out of sync during transitions; instead of moving in unison, the rear tires' reactions lagged behind the front tires, resulting in a detached sensation being transmitted to the drivers.

The underinflated tires delivered acceptable steady-state cornering force once they stabilized on our test track's skid pad, but the car was uncooperative anytime it was asked to change directions. It proved to be over 2 seconds slower around our test course (2 seconds represents about a 7% loss of handling performance).

In other words, the performance that tire manufacturers build in, low tire pressures can take away.

Adjust your tire pressures as indicated on the vehicle tire placard or in the owner's manual. Check you inflation pressures at least once a month and before highway trips.


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#3174 - 10/19/09 09:19 AM Re: How to Read Speed Rating, Load Index, Service Descriptions [Re: teamzr1]  
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Air Pressure, Temperature Fluctuations

Your tires support the weight of your vehicle, right? Well they don't! It's the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.

However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time, changes in ambient temperatures or a small tread puncture have not caused it to change.

The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle's recommended cold tire inflation pressure.
This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it.

Since air is a gas, it expands when heated and contracts when cooled. In most parts of North America, this makes fall and early winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tires' inflation pressure is going down!

The rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

In most parts of North America, the difference between average summer and winter temperatures is about -50 Fahrenheit...which results in a potential loss of about 5 psi as winter's temperatures set in. And a 5 psi loss is enough to sacrifice handling, traction, and durability!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20 Fahrenheit.
This means that after setting tire pressures first thing in the morning, the vehicle's tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade).
While that is expected, the problem is when you set your vehicle's tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun's radiant heat will artificially and temporarily increase tire pressures.

We put some of these theories to the test at the Tire Rack. First, we mounted two tires on wheels. We let them sit overnight to equalize and stabilize their temperatures and pressures.

The following morning we set them both to 35 psi. One tire and wheel was placed in the shade while the other was placed directly in the sun. We then monitored the ambient temperatures, tire temperatures and tire pressures through the day. As the day's temperatures went from 67 to 85 Fahrenheit, the tire that was kept in the shade went from our starting pressure of 35 psi to a high of 36.5 psi.
The tire that was placed in the sun and subject to the increase in ambient temperature plus the sun's radiant heat went from our starting pressure of 35 psi to a high of 40 psi.
In both cases, if we had set our tire pressures in the afternoon under the conditions of our evaluation, they would have been between 2 and 5 psi low the following morning.

Next we evaluated the affects of heat generated by the tire's flexing during use. We tried to eliminate the variable conditions we might encounter on the road by conducting this test using our "competition tire heat cycling service" that rolls the tires under load against the machine's rollers to simulate real world driving. We monitored the changes in tire pressure in 5-minute intervals.

The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation.

Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. This means that even a short drive to inflate your tires will result in tires that will probably be under-inflated by a few psi the following morning.

Add all of these together, and you can understand why the conditions in which you set your vehicle's tire pressures are almost as important as the fact that you do set it.

It's important to remember that your vehicle's recommended tire pressure is its cold tire inflation pressure. It should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it.

And by the way, if you live in the North and park in an attached or heated garage you will lose pressure when you leave its warmth and venture into the real world outside during winter. Add 1 psi cold pressure tire pressure to compensate for each 10 Fahrenheit temperature difference between the temperature in the garage and outside.


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