General Information

P-metric designation (e.g., P245/50ZR16)


[Code][Width]/[Aspect Ratio][Speed Rating][Construction][Size]

P = Passenger
LT = Light truck
T = Temporary
LTP = Light truck, personal use
C = Commercial

Width at widest point, sidewall to sidewall, in millimeters

Aspect Ratio:
Tire height in mm, measured tread to bead, divided by width above, multiplied by one hundred.

Speed ratings:
(The maximum speed the tire can maintain at its maximum load rating)
Q - 100 mph
R - 106 mph
S - 112 mph
T - 118 mph
U - 124 mph
H - 130 mph
V - 149 mph
W - 168 mph (A "Z" rating is featured in the size code)
Y - 186 mph (A "Z" rating is featured in the size code)

R - Radial ply
B - Bias belted
D - Diagonal bias

The diameter of the wheel the tire is designed to be mounted on, in inches.

Other information:

Maximum Air Pressure
This is the max pressure the tire is designed to hold (note this is more than the recommended inflation pressure)
Maximum Load
Weight the tire can support. Multiplying this number by the number of tires results in the total weight they can bear, which may differ from the manufacturer's recommended gross vehicle weight due to chassis limitations.
Load Index
A code that indicates the maximum weight the tire can carry at its speed indicated by its speed rating
Tells the number and type of plies used in the construction of the tread and sidewall
DOT Serial Number
Identifies the plant, manufacturer, and date of production. The last three numbers indicate the week and the last digit of the year the tire was made.
Tread Wear Index
A rating of the tire's resistance to wear. 100 is the reference. A 250 rating indicates 2.5 times the wear resistance of the reference tire. This number cannot be accurately translated to mileage.
Traction Index
A rating of the tire's ability to stop on wet pavement. There are three grades, A, B, and C, with A being the best. This index does not reflect the tire's traction performance on dry, snowy, or icy pavement.
Temperature Index
A rating of the tire's ability to dissipate and resist heat. There are three grades, A, B, and C, with A being the best. To earn an A rating, the tire must withstand a thirty-minute run at 115 mph without failing.

From: Jeff Berton

Tire Pressure

Under-inflation and over-inflation both warp the tread surface and reduce the contact patch of the tire on the road. Thus, more of the load of the vehicle is carried by a smaller percentage of the tire. Usually, the wear created by either of these conditions can be spotted with a visual inspection. Over-inflation leads to premature wear of the center of the tread. Under-inflation will wear both outside edges of the tread before the center. Premature wear on the inside of the tire only indicates an alignment problem (toe-out). Premature wear on the outside of the tire only indicates either exuberant driving habits, or an alignment problem (toe-in). Cupping of the tire indicates a suspension problem or severe misalignment.

What the Goodyear dealer does not appear have told you, is that premature wear of only one of the rear tires on V6 Fiero, tends to indicate a rather severe problem with the "Fuel System and Clutch Linkage Actuator Assembly" and you may wish to take a closer look at this device. A problem with this can best be spotted by a bluish haze behind the car at very slow speeds that will smell much like a burnt rubber band. (;-)

There are benefits to either non-spec inflation scenario in limited circumstances. When stuck in the snow or on beach surfaces it is desirable to temporarily lower the air pressure in the tire at the expense of tire wear. The additional sidewall flex allows the tire to "float" more readily on top of the surface, instead of digging down into it. Over-inflation is desirable in racing circumstances (again, at the cost of tire wear) since this increases the rigidity of the sidewall and reduces the tires tendency to flex during hard cornering. Theory here, is that even though the contact patch is reduced in a straight line, during hard cornering, the extra rigidity actually allows more of the tire to remain in contact with the ground since it doesn't roll under.

I should note here, that you should NEVER exceed the max inflation pressure on the sidewall of the tire. This is the tire manufacturer's tested limit of inflation and to exceed this could result in sudden tire failure. Depending on how much you exceed the max pressure of the tire, this could happen very quickly. Anyone ever had a bicycle tire blow up in their face before?

For daily driving, 40psi is *much* too high. This is probably too high even for an autocross, although I'll defer to the more experienced racers on the list. In any event, you're consuming tread at an alarming rate, and unless you only drive with the door handles dragging on the ground, you're not getting anything significant back in terms of handling.

Conversely, the door jamb pressures are biased a little towards the comfort end of the spectrum so you may wish to increase the pressure in the tires *a little* bit. I run mine at 33 psi on both ends. Plenty of sidewall stiffness for exuberant (if not racing) driving, and not so much as to significantly increase the wear on the tire.

From: Mark Madden

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