Sidewall Markings

There is a lot of information on the sidewall of a tire. Typically, you'll
find UTQG ratings for treadwear, traction and temperature, the size of the tire,
the load rating index number with a speed rating index, the construction type
(bias or radial), the D.O.T. (Dept. of Transportation) compliance code,
construction details, and of course, the make and model of the tire. On some
tires used as original equipment, you may also find a marking that indicates its
OE status. Porsche uses an N-0 or N-1 designation, BMW uses a star on some O.E.
tires and General Motors uses a "TPC" code. Light Truck tires are sometimes
marked with an LT for "Light Truck" before the size, passenger tires are often
marked with the letter P for "Passenger" before the size. Passenger tires of the
same size with or without the P are virtually interchangable.

UTQG Ratings

The Department of Transportation requires each manufacturer to grade its
tires under the Uniform Tire Quality Grade (UTQG) labeling system and establish
ratings for treadwear, traction, and temperature resistance. These tests are
conducted independently by each manufacturer following government guidelines to
assign values that represent a comparison between the tested tire and a control
tire. While traction and temperature resistance ratings are specific performance
levels, the treadwear ratings are assigned by the manufacturers following field
testing and are most accurate when comparing tires of the same brand.

Treadwear

Treadwear receives a comparative rating based on wear rate of the the tire in
field testing following a government specified course. For example, a tire grade
of 150 wears 1.5 times as long as a tire graded 100. Actual performance of the
tire can vary significantly depending on conditions, driving habits, care, road
characteristics, and climate.

Traction

Straight-a-head wet braking traction has been represented by a grade of A, B,
or C with A being the highest. In 1997 a new top rating of "AA" has been
introduced to indicate even greater wet braking traction. However, due to its
newness, this grade will probably be applied initially to new tire lines as they
are introduced and later to existing lines which excel in wet braking, but had
been limited to the previous top grade of "A". Traction grades do not indicate
wet cornering ability.

Temperature

Temperature resistance is graded A, B or C. It represents the tire's
resistance to the heat generated by running at high speed. Grade C is the
minimum level of performance for all passenger car tires as set under Federal
Motor Vehicle Safety Standards. This grade is established for a tire that is
properly inflated and not overloaded.

Note: UTQG ratings are not required on winter and light truck sized
tires.

Calculating Tire Dimensions

Modern metric tire dimensions  Example...185/60R14 85H or 185/60HR14

The first number is the width of the tire in millimeters, measured from
sidewall to sidewall. To convert to inches, divide by 25.4 In the example above,
the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to
width. In the example above, the tire is 7.28" wide, multiply that by the aspect
ratio to find the height of one sidewall. In this case, 185x0.60=111mm or
7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and
multiply by 2,(remember that the diameter is made up of 2 sidewalls, the one
above the wheel, and the one below the wheel) and add the diameter of the wheel
to get your answer.

Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

Tire Size Conversion Chart

While today's P-metric passenger tire sizes have existed since the early
1980's, restoring classic muscle cars and ponycars has kept yesterday's Numeric
and Alpha Numeric tires from disappearing. The chart below is intended to help
you determine their equivalent P-metric tire sizes.

Two versions of numeric tire sizes were used as original equipment on
vehicles between 1949 and 1970. The early numeric tires had the equivalent of a
90-series aspect ratio, while later tires offered a "lower" profile equivalent
to a 80-series. These tires typically featured tread widths ranging from 3.5"
for the smallest 13" rim diameter tires to about 5.5" for the largest 15" rim
diameter tires.

To convert Numeric sizes to today's P-metric sizes, it is important to
remember that early cars were not only equipped with narrow tires, they were
equipped with narrow wheels as well. In most cases, numeric tires should be
replaced with today's 80- or 75-series sized tires. This is especially important
if the original wheels are to be used. Today's lower profile sizes will usually
result in too wide a tire with too much gap between the wheel well opening and
the top of the tire.

For example: The 1965 Ford Mustang's 6.95-14 would be replaced with a
P185/75R14.

Tire Size

Overall Diameter

Section Width

Load Capacity

6.95-14

25.3"

7.0"

1230 lbs @ 32 psi

P185/75R14

25.0"

7.2"

1290 lbs @ 35 psi

Alpha numeric tire sizes were introduced as original equipment in the late
60's and became widely used in the early 70's. These tires were identified with
a letter which indicated the tire's load capacity, followed by a "R" if radial
ply construction, the tire's aspect ratio and wheel diameter. So while G78-15
(28.01"), G70-15 (27.5") and G60-15 (26.4") sized tires are all rated to carry
the same load, their different aspect ratios resulted in tires with the overall
diameters indicated above.

To convert Alpha Numeric tires to P-metric sized tires, it is important to
identify the original tire's aspect ratio. The 78-series Alpha Numeric tires
should be replaced with today's 80-, or 75 series tires. If the vehicle was
equipped with the low profile 70-, 60- or 50-series sizes, the P-metric
substitution should be selected from the P-metric size column that offers the
equivalent aspect ratio as the existing tire.

For example: The 1970 Chevrolet Camaro Z-28 F60-15 would be replaced with a
P235/60R15 (selected from the P-metric 60-series column).

Tire Size

Overall Diameter

Section Width

Load Capacity

F60-15

25.9"

9.2"

1500 lbs @ 32 psi

P235/60R15

26.1

9.5"

1642 lbs @ 35 psi

1949 to 1964

1965 to 1970

1970 to 1980

1980 -
On

NUMERIC

ALPHA NUMERIC

EUROMETRIC

P-METRIC

EUROMETRIC or
P-METRIC

"90" to
"80"-Series

78 to 50-Series

"82"-Series

80-, 75-series

70-series

65-series

60-series

50-series

145R10

165/70R10

145R12

P145/80R12

165/70R12

6.00-12

155R12

P155/80R12

165/65R13

145R13

155R13

P155/80R13

175/70R13

195/60R13

215/50R13

5.60-13

6.00-13

A

165R13

P165/80R13

185/70R13

205/60R13

6.50-13

B

175R13

P175/80R13

C

P185/80R13

7.00-13

D

185R13

195R13

175/65R14

6.00-14

6.45-14

155R14

175/70R14

185/65R14

B

165R14

P175/75R14

185/70R14

195/65R14

205/60R14

6.50-14

6.95-14

C

175R14

P185/75R14

195/70R14

215/60R14

D

7.00-14

7.35-14

E

185R14

P195/75R14

205/70R14

225/60R14

245/50R14

7.50-14

7.75-14

F

195R14

P205/75R14

215/70R14

235/60R14

8.00-14

8.25-14

G

205R14

P215/75R14

225/70R14

245/60R14

265/50R14

8.50-14

8.55-14

H

215R14

P225/75R14

9.00-14

J

225R14

9.50-14

L

5.60-15

A

155R15

P155/80R15

185/65R15

195/60R15

6.00-15

B

165R15

P165/80R15

185/70R15

195/65R15

205/60R15

225/50R15

6.50-15

6.85-15

C

175R15

195/70R15

205/65R15

215/60R15

D

7.35-15

E

185R15

P195/75R15

205/70R15

215/65R15

225/60R15

6.70-15

7.75-15

F

195R15

P205/75R15

215/70R15

235/60R15

8.15-15

7.10-15

G

205R15

P215/75R15

225/70R15

235/65R15

245/60R15

265/50R15

8.25-15

8.45-15

7.60-15

H

215R15

P225/75R15

235/70R15

255/60R15

275/50R15

8.55-15

8.00-15

8.85-15

J

225R15

P225/75R15

235/70R15

265/60R15

8.20-15

9.00-15

K

9.15-15

L

235R15

P235/75R15

255/70R15

275/60R15

295/50R15

N

305/50R15

• Listing in chart does
not imply complete interchangeability.

• When changing tire
sizes, dimensional clearances must be checked.

Each row
of the above chart is roughly the same diameter tires, but the width varies by
column, and the diameters are not exact matches.

Diameter Comparison

While this chart certainly doesn't guarantee a specific tire will fit your
vehicle, it can give you an idea of the tire sizes with the same diameter.

Approx.
Tire
Diameter

P-metric and
European Metric

LT-metric

Light Truck

Light Truck

(in Inches)

75-series and Higher

70- and 65-series

60-series and Lower

85-series

75-series and Lower

Flotation

Numeric

34 1/2

35X12.50R15LT

33 1/2

LT255/85R16

33

LT285/75R16

32 1/2

33X12.50R16.5LT

33X12.50R15LT

31 1/2

P275/70R16

LT235/85R16

LT265/75R16

32X11.50R15LT

7.50R16

30 1/2

P265/75R15

265/70R16

285/60R17

LT215/85R16

LT285/60R17

31X10.50R16.5LT

9.50R16.5LT

LT245/75R16

31X11.50R15LT

LT325/60R15

31X10.50R15LT

30

P255/70R16

P275/60R17

29 1/2

P245/70R16

265/60R17

LT225/75R16

30X9.50R15LT

8.75R16.5LT

7.00R15

29

205R16

255/65R16

255/55R18

LT235/75R15

205/80R16

P235/70R16

255/60R17

P235/75R15

P255/70R15

28 1/2

P225/75R15

P225/70R16

LT225/75R15

29X9.50R15LT

8.00R16.5LT

P245/70R15

28

215/70R16

P255/55R17
P275/60R15

27 1/2

P215/75R15

P225/70R15

LT215/75R15

27

P205/75R15

P215/65R16

255/50R17

LT205/75R15

P305/50R15

P255/60R15

26 1/2

P195/75R15

P225/70R14

P295/50R15

LT195/75R15

27X8.50R14LT

P245/60R15

26

P205/75R14

P215/65R15

P235/60R15

185R14

P215/70R14

25 1/2

P195/75R14

P205/70R14

P225/60R15

P-Metric and Euro Metric Sizing

What's the difference between the tire sizes of P225/60R16 and 225/60R16? The
obvious answer is the "P" in front of the first size, but just what does the "P"
stand for and what does it tell us about the tires?

P-metric sized tires are the ones with the "P" at the beginning of the tire
size, (such as P225/60R16 listed above). They were introduced in the United
States in the late 70s and are installed on vehicles primarily used to carry
passengers including cars, station wagons, sport utility vehicles and even light
duty pickup trucks. Their load capacity is based on an engineering formula which
takes into account their physical size (the volume of space for air inside the
tire) and the amount of air pressure (how tightly the air molecules are
compressed). Since all P-metric sizes are all based on the formula for load,
vehicle manufacturers can design their new vehicles (weights and wheel well
dimensions) around either existing or new tire sizes.

Metric or Euro metric sized tires are the ones without the "P" at the
beginning, (such as 185R14 or the 225/60R16 listed above). Using metric
dimensions to reflect a tire's width actually began in Europe in the late 60s.
However, since Euro metric sizes have been added over time based on the load and
dimensional requirements of new vehicles, the tire manufacturers designed many
new tire sizes and load capacities around the needs of new vehicles. Not quite
as uniform as creating sizes using a formula, but they got the job done.

Euro metric and P-metric tires in the same size (i.e. P225/60R16 &
225/60R16) are equivalent in their dimensions with just slight differences in
their load capacity calculations and inflation pressure tables. So if Euro
metric and P-metric tires have the same numeric size, the same tire performance
category and the same speed rating, the two are considered equivalent and
interchangeable if used in axle pairs or sets of four. Simply continue to follow
your vehicle manufacturer's recommended inflation pressures provided in the
vehicle's owner's manual or on the vehicle tire placard (usually found on the
door jamb or on the glove box or counsel door) for either size tire.

The format, and possibly the content of this article has been
modified.

Special Thanks to The Tire Rack for this article.