Pontiac FIREBIRD 1972 Service Manual page 1241

10-4 1972 PONTIAC SERVICE MANUAL
GENERAL DESCRIPTION
All models except X Series have bias-belted tires as stand-
ard equipment. These tires have two polyester cord bias
plies in the tire body, plus two glass fiber plies, or "belts",
under the tread only (Fig. 10-3). The advantages of the
bias-belted tire over the bias ply tire are increased tire life,
greater traction, and better resistance to road hazards due
to greater strength.
X Series models have bias-ply tires as standard equipment
with bias-belted optional. The Sprint option also includes
bias-belted tires.
Belted tires should not be mixed with conventional tires
on any car as a brake pull or a drifting under acceleration
will result.
All models use steel drop center rim wheels secured by
right-hand threaded nuts on either side of the vehicle.
Wheel sizes are as follows: A, F and G Series
-
14
x
6",
14
X
7" and 15
X
7"; B Series
-
15
x
6"; X Series
-
14 X 5" and 14 X 6". Various wheels can be identified by
a letter code stamped on the rim near the valve hole (Fig.
10-13).
TIRE SIZE A N D LOAD RATING
Tire sizes and load ratings are indicated by a combination
of numbers and letters such as G78 X 14, load range B.
The first letter designates the load the tire will carry at a
given inflation pressure. The "higher" the letter, the big-
ger the tire and so the greater the load capacity. Fig. 10-4
gives approximate size comparisons between conventional
and wide tread tires.
The first set of numbers (78) denotes the ratio of tire
height to width. Height di;idkd by width equals section
ratio. For example, a G78 tire is 78% as high as it is wide.
The lower the number, the wider the tire.
TIRE BODY CORDS
/
TREAD RUBBER
FIBERGLASS LAYERS
Fig.
10-3
Bias-Belted Tire Construction
I
1
7.35, 7.75, 8.25,
etc.
1
83
(Section Ratio)
Conventional
2 Ply-4 P l y
R a t i n g
4 Ply-8 P l y
R a t i n g
Fig. 10-4 Tire Size Comparisons
Wide T r e a d
L o a d Range
B
L o a d Range
D
The second set of numbers (14) shows the inner diameter
of the tire.
Load range replaces the ply-rating system (Fig. 10-4).
Load range B and load range D tires will carry the same
load at the same pressure. However, load range D tires
may be inflated to higher pressures. At these higher pres-
sures they will carry a greater load.
NOTE: Use of optional, larger, or hkher load
range tires does not permit an increase in the
maximum vehicle load.
Normal passenger or cargo loads do not appreciably in-
crease the load carried by the front tires. The rear tires
carry most of this load. Rear tire pressures of load range
B tires may be increased up to 32 psi and load range D tire
pressures increased up to 40 psi for extra tire load
capacity.
The ride quality of tires at increased pressures may be
hard compared to normal pressures. It is important that
tire pressures be maintained at recommended values when
car is carrying normal loads.
NOTE: White sidewall tires should have the
protective coating washed o f f before being
placed in service. This coating is not as fixible
as rubber and therefore checks, which may in-
troduce sidewall check if not removed.
INFLATION OF TIRES
Correct inflation pressure is one of the most important
elements of tire care. The inflation pressure recommended
for any model of car is carefully calculated to give a cor-
rect balance of satisfactory ride, stability, steering, tread
wear, cord life and resistance to stone bruises.