Tire Size FAQ


Generic Auto Tire FAQ
compiled by Bill Del Vecchio
Generic Auto Tire FAQ

Introduction:

This FAQ (Frequently Asked Questions) was assembled to address some of the most common inquiries received in the Compuserve CARS forum regarding the basics of tire selection, sizing and shopping. It attempts to give an overview covering typical consumer concerns when shopping for DOT-approved passenger car tires in the U.S. and/or Canada. Variations not covered include older, historical standards, non-US designations, truck tires and other specialty applications.

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Table Of Contents

1. What do all those sidewall markings mean?
A. Size (i.e. 205/60-15)

B. Speed Rating (i.e. H, V, Z)

C. Load Index (i.e. 89, 92, 94)

D. UTQG Ratings (Temperature, Traction, Treadwear)

E. M&S Designation

F. Max. Load

G. Max. Press

H. Type of Construction

I. DOT Serial Number

J. Manufacture Date (i.e. 134) 2. Tire Care: Inflation & Rotation

3. Alternate Sizing
A. Why Do It?

B. How do I calculate Tire Height and Rotations Per Mile?

C. Can I put wider tires on my stock rims?

D. What's a Plus One, Plus Two or Plus Three conversion? 4. Buying Tires
A. What brands and models are best?

B. What factors are important to consider when choosing tires?

C. How do I choose between Performance, Touring and All Seasons?

D. Which All Season tires are best in the snow?

E. Where should I buy my tires? 5. What about snow tires?

6. What about buying new wheels with my tires?

7. What about tires for my SUVs or 4x4?

7. FAQ Sources, Bibliography and Revision History

8. Copyright & Other Important Information

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1.A. How do I read a tire's sidewall to tell its size?

A tire's size is expressed in the format WWW/AA-DD (i.e. 205/60-15) where WWW is the tire's sidewall-to-sidewall Width in millimeters (205), AA is the Aspect ratio or profile of the tire which specifies the tire's height as a percentage of its width (60% of 205 = 123mm) and DD is the diameter of the wheel that the tire is mounted on in inches (15 inches). If the size is shown as P205/60R15, the "P" stands for Passenger and the "R" is for Radial ply construction. Note that the width does not measure the tread, which can vary significantly between tires with the same nominal width, even within product lines from the same manufacturer.

What about an older tire that doesn't list an aspect ratio?

For an older tire without an aspect ratio (i.e. 175R13), it's generally assumed to be a 78 or 80 series tire, (i.e. 175/78R13 or 175/80R13). Note that for some European tire models, the default ration is 83. Fortunately, the practice of not listing the aspect ratio is getting less and less common.

1.B. Speed Rating

The speed rating was traditionally shown as a part of the tire's size, i.e. "205/60VR15". Since the inclusion of Load Ratings (see 1.C below), many manufacturers are now showing the speed rating after the size in combination with the load rating, i.e. "205/60R15 92V". Commonly used speed ratings include:

Certified

Rating Top Speed

N 87 mph

Q 100 mph

S 112 mph

T 118 mph

U 124 mph

H 130 mph

V 149 mph*

Z over 149 mph

W 168 mph

Y 186 mph

* Originally, V was "over 130mph". W and Y ratings are relatively new, hence the redundancy with Z, which will probably be dropped at some point in the future.

1.C. Load Index

The Load Index indicates the maximum weight the tire can carry at the maximum speed indicated by its speed rating. Some sample Load Rating Indices:

Rating Capacity (lbs)

75 853

82 1,047

85 1,135

87 1,201

88 1,235

91 1,356

92 1,389

93 1,433

105 2,039

1.D. UTQG (Uniform Tire Quality Grading)

...rates three aspects of tire performance: Temperature, Traction and Treadwear. These grades are usually located together on the tire sidewall. Tire manufacturers do their own UTQG testing and assign their own grades. Some variations in testing are inherent due to the lack of controls. FWIW, experts usually agree that Treadwear is consistent within a given manufacturer's product lines.

Temperature: A/B/C

Shows the tire's ability to dissipate heat and its resistance to heat generation. "C" means that the tire meets minimal federal standards measured in laboratory testing, while "B" and "A" indicate increasingly better performance above the government minimum.

Traction: A/B/C

Grades straight-line wet braking performance. A is best, C is worst.

Treadwear: Numeric Grade

Rates the tire against a control standard with a defined rating of 100 when run on a test course. A rating of 300, for instance, indicates that the tire will give three times the mileage of the control tire. One rated 60 would be expected to wear out in 60% of the control tire's mileage.

1.E. M&S Designation

Indicates an all-season tire designed for Mud & Snow use. Note that this does *not* mean that this is a "snow tire" (see Section 6).

1.F. Max. Load

This is the maximum static weight, usually in both pounds and kilograms, that the tire can support.

1.G. Max. Press.

The Maximum Pressure (usually in PSI and kPa) that the tire is designed to handle. This is *not* the recommended pressure, which is set by the car manufacturer and stated in the owners manual as well as on a sticker in the door jamb or glovebox lid (see Section 2).

1.H. Construction

The number and composition of the tread and sidewall plies are listed on the sidewall, for example:

Tread Plies: 2 polyester cord + 1 steel cord + 1 nylon cord

Sidewall Plies: 2 Polyester Cord

1.I. DOT Serial Number

All tires approved for street use in the U.S. by the Department of Transportation will display a DOT Serial Number.

1.J. Manufacture Date

All tires are coded to indicate the week of manufacture. Look for a three digit number following the DOT Serial Number. The date code will be stamped rather than molded. The first two digits are the week of the year in which the tire was manufactured (01 thru 52), followed by the final digit from the year of manufacture. A tire stamped "134" was manufactured in the 13th week (the week of April 4th in this case) of 1994.

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2. Tire Care: Inflation & Rotation

What inflation pressure should I use in my tires?

As stated in 1.G. above, recommended tire pressures are set by the car manufacturer, not by the tire manufacturer. They're stated in the owners manual as well as on a sticker in the door jamb or on the glovebox lid. It's important to check pressures frequently; at least once a month - more often if you find that you're losing pressure. For consistency, always measure pressures before driving when the tires are cold (parked for at least four hours; preferably overnight).

Tire pressure affects your car as follows:

Lower Pressures Higher Pressures

Ride more comfortable stiffer

Handling less precise better feel & turn-in

Wear more on edges more in the center

Gas mileage lower higher (less rolling resistance)

Tracking steady tends to follow grooves

Many car manufacturers are conservative (low) with their recommended pressures to maintain a comfortable ride. Drivers who push their cars fairly hard usually prefer a slightly higher setting. If you want to experiment, 32 all around is a good starting point - begin there adjust to your own preferences.

The outside temperature affects your tire pressures; a general rule of thumb is that a 10 degree fahrenheit change will change tire pressure by 1 psi. When temperatures are fluctuating, check your pressures more often. And if you check them in a heated garage, adjust for the colder outside temps.

How about rotation?

Recommendations for tire rotation patterns and mileage intervals vary; check your car's Owners Manual and the tire manufacturer's recommendations for some guidelines. As a general rule, rotation is advised at least every 10,000 miles. It may be prudent to rotate more often if you're using ultra high performance tires that wear quickly, or if your car's suspension settings tend to wear the tires at one end of the car at a much more rapid pace. Many people find it convenient and a helpful reminder to rotate their tires along with their oil change schedule, every 7,500 miles for the typical car or every other change for those with more frequent changes. YMMV!

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3.A. Alternate Sizing - Why Do It?

Variations from the OEM tire size are usually done to improve a car's handling and performance. Except in the case of snow tires (see Section 5), a common goal is to get a wider tire and/or shorter sidewalls without changing the overall diameter of the tire. Wider tread width changes the tire's contact patch and can lead to a big improvement in handling. In general, more rubber on the road provides a better grip. A shorter sidewall (lower aspect ratio) provides less flex which can improve turn-in, responsiveness and stability. Keeping the overall tire height as close to stock as possible reduces the chance that changes will impact the accuracy of the car's speedometer and odometer, overall gearing, suspension dynamics and introduce potential clearance problems.

Note Well: With any change from the OEM size, clearance between the tire, fender, fender wells and suspension components is a vital concern. Also, expect variations from calculated tire dimensions; whenever possible, measure the actual, mounted tire.

3.B. How do I calculate Tire Height and Rotations Per Mile?

A tire's theoretical height in inches is calculated as follows:

Height = (Width x (Ratio/100) x .03937 x 2) + Rim Diameter

The width multiplied by the aspect ratio over 100 gives the height in millimeters, multiplying by .03937 converts to inches, and doubling this accounts for the fact that there's tire at the bottom and the top of the rim.

Rotations Per Mile (RPM) can be calculated as follows:

RPM = (inches per mile) / Tire Circumference

= (5280*12) / (Height*3.1416)

= 20,168 / Height

Some sample calculations of tire height and RPM are shown below:

Height RPM Error @60MPH Note:

205 60 15 24.69 817 - 60.0 Stock size

225 55 15 24.74 815 0.2% 60.1 Wider tire on stock

225 50 16 24.86 811 0.7% 60.4 Plus One

245 45 16 24.68 817 0.0% 60.0 Plus One

245 40 17 24.72 816 0.1% 60.1 Plus Two

245 35 18 24.75 815 0.3% 60.2 Plus Three

185 65 15 24.47 824 -0.9% 59.5 Snow tire alternative

It may be helpful to set up a simple spreadsheet to determine options. Height and RPM calculations are given above, while the error and speed at an indicated 60mph are calculated from the RPM as compared to the stock size. An error of less than 1% almost always indicates a very good match.

3.C. Can I put wider tires on my stock rims?

The cheapest way to pick up additional performance is usually to get a slightly wider tire with a lower aspect ratio for use on the stock rim. For example, a stock 205/60-15 tire can be replaced by a 225/50-15 with almost no variation in overall diameter. Note well that in addition to the clearance considerations listed above, the width of the stock rim also needs to be able to accommodate the wider tire. A 6" wide rim easily accommodates a 205/60 tire, but is at the low end of recommended rim widths for most manufacturers' 225/50 tires. Wherever possible, seek advice from the tire manufacturer or other owners of your car to see what width tire your stock rims can accommodate.

3.D. What's a Plus One, Plus Two or Plus Three conversion?

Also shown as "+1", "+2" or "+3", these designations indicate switching from stock rims to rims of 1, 2 or 3 inch larger diameter. Going to a larger diameter rim while keeping overall tire height about the same can allow for a significantly wider, shorter tire which can have a dramatic effect on a car's handling, ride and appearance.

As shown in 2.B. above, potential replacements for a stock 205/60-15 can range from a prudent +1 combination like 225/50-16 to a rather outrageous 245/35-18 for a +3 conversion. Note that price increases are not linear; although 18 and even 19 inch tires and rims are available, they can cost several times what a 15 or 16 inch setup will run.

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4.A. Buying Tires - What brands and models are best?

There are no "best" choices. What's good for you is highly dependent upon your car and your preferences. With that in mind, here's a broad listing of some of the tires most frequently recommended by users of the CARS forum, broken down by type:

Ultra High Perf: Pirelli PZero, Bridgestone Expedia

High Performance: Dunlop SP8000* and D40M2*, Yokohama AVS Intermediate* and A509*, Pirelli P700Z, Bridgestone RE71, B.F. Goodrich Comp T/A 3, Goodyear GS-C

Touring: Pirelli P4000 M&S, Bridgestone Turanza M&S, Yokohama A378

All Season: Dunlop D60A2*, Pirelli P500, Yokohama AVS U+4, Michelin XGT V4

Race/Autocross: Yokohama A008 RS* and A008 RS II, B.F. Goodrich Comp T/A R1 (226 and 230 compound), Goodyear GS-CS

* CARS All-Stars - most frequent positive feedback from satisfied users

4.B. What factors are important to consider when choosing tires?

The best tire depends upon a wide variety of factors, and the more specific you can be when asking a pro for advice, the better advice you're likely to receive:

What car, truck or minivan do you drive? What's the stock tire size? Are you considering a change? How do you drive? Is your driving style aggressive, passive or in-between? Where do you drive? City, highway, back roads, mountains, at the track? In what kind of weather? What's your regional climate? Is the car used year round? How much snow or heavy rain do you get? What's most important to you? How would you prioritize traction, ride comfort, treadwear, steering response, handling and noise? What's your price expectation? How important is value compared to the tire performance items you prioritized above? Do you have any particular brands or models in mind? Have you had good or bad experiences with any models in the past? How did your current tires perform?

4.C. How do I choose between Performance, Touring and All Seasons?

Tough question! Here are some random thoughts and notes to help:

Ultra High performance tires provide more traction and performance than can be used safely on the street. They are a waste of money for most drivers and most cars unless they are used as a dual purpose street/racing tire. Tire life under 10,000 miles should be expected on some car models. High Performance tires also provide more traction and performance than a responsible driver should be using on the street. But by doing so, they increase the safety margin in emergency handling maneuvers. There are some very competitive, cost-effective models in this classification; it isn't necessary to spend big bucks to get a dramatic performance and safety improvement over original equipment tires. The "Touring" designation is relatively new. Definitions may vary between manufacturers, but most are marketed as competent year 'round performers that are better than an all-season for performance driving and better than a performance tire in slippery conditions. A true All Season tires has a "M+S" (mud & snow) designation on its sidewall. All Seasons usually do much better than performance tires in mud, snow or slush, but it's something of a misconception that they also do better in wet conditions. In tire tests (see Section 7), performance tires did better in wet handling and braking than All Seasons. In general, All Seasons are a compromise solution that let you use the same tires year-round, even where snow is expected. Using performance tires and a set of snow tires (see Section 5) is recommended by many CARS forum regulars.

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5. What about snow tires?

For snow tires, narrower and taller usually works better than the stock tire size, and some experts recommend considering a "Minus One" conversion, especially for cars equipped with wide tires. The sample calculations in 2.B. above are also used for determining narrower snow tires sizes and for Minus One conversions, i.e.:

Height RPM* Error @60MPH Note:

275 40 17 25.66 786 - 60.0 Stock size

205 60 16 25.69 785 0.1% 60.1 Minus One

Height RPM* Error @60MPH Note:

205 50 15 23.07 874 - 60.0 Stock size

185 55 15 23.01 876 -0.3% 59.8 Narrower Snow Tire on Stock Rim

For serious driving in the snow, regular or all-season tires are no match for a good set of four snow tires. Among CARS forum members, the most frequently mentioned models are the Pirelli 190P & 210P and the Bridgestone Blizzak. The Pirellis are a very good all-around choice for a snow tire with reasonable dry performance and treadwear expectations, and the new Pirelli S/P is a good choice for a severe duty, studdable tire.

The Blizzaks were only recently introduced, but already have a lot of fans. They provide great traction on slippery surfaces, but their dual compound tread design means that they may not be a good choice for all situations. Once the Blizzak's tread is worn below 55%, it is designed to work as an all-season tire rather than a snow tire. This may work well for someone who puts high annual mileage on their car, expects to replace their tires yearly and prefers not to have to keep a second set of tire. By purchasing a new set of Blizzaks in the late fall, you can insure optimal snow traction throughout the winter. As the tires wear down, you get a competent all-season to use until the next replacement cycle. As an alternative, someone who puts lower mileage on their car can keep a set of Blizzaks through several winters by installing them just before snow starts and removed in early Spring.

Nokia Hakkapeliittas deserve a special mention as perhaps the premium snow tire which is used by more rallyists than any other brand. Because of limited distribution, they can be difficult to find, but several forum members have recommended Greer Enterprises in Milwaukee (414-744-0996) as a source for Nokia tires.

Do I *need* four snow tires?

The question of 2 vs. 4 snows has been raised several times. While convent- ional wisdom used to be that putting snows on the drive wheels was sufficient, more recent recommendations have been to use all four. Why? The primary concern is that snows and all-seasons have dramatically different handling characteristics.
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