A-1 Truck Warehouse of Kansas FAQ's
This FAQ (Frequently Asked Questions) was assembled to
address some of the most common inquiries received 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.
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
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.
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.
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.
4.A. Buying Tires - What brands and models are best?
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?
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.
4.C. Which All Season tires do best in the snow?
Richard Engel [76614,3653] of The Tire Warehouse in Edmonton, Alberta
offered that the Dunlop D65, Axiom and Sport 4000, Pirelli P300 and Yokohama
Y370/Y378 are several top performers in the snow, but he's quick to remind
everyone that to get the best traction, you need a set of winter tires.
4.D. Where should I buy my tires?
Perhaps the best way to buy tires is to contact
us today.
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. 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.
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.
|
