Did some research on the net. Here I'm pasting some useful data from it.

How to read your Tyre - The basics

215 / 45 / R 17 91 Y

215 - This is the width in mm of the tyre from sidewall to sidewall when it's unstressed and you're looking at it head on (or top-down). This is known as the section width.

45 - This is the ratio of the height of the tyre sidewall, (section height), expressed as a percentage of the width. It is known as the aspect ratio. In this case, 45% of 215mm is 96.75mm - the section height.

R - Stands for Radial for tyre construction type. Other types are diagonal bias (D) or bias belted (B) construction.

17 - Rim diameter

91 - Load index

Y - Speed Rating

Speed Ratings

All tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute endurance without coming to pieces and destroying itself, your car, the car next to you and anyone else within a suitable radius at the time.

Speed Rating - Max Speed Capability (Km/h)
L - 120
M - 130
N - 140
P - 150
Q - 160
R - 170
S - 180
T - 190
U - 200
H - 210
V - 240
W - 270
Y - 300
Z - 240+

Load Index

The load index on a tyre is a numerical code associated with the maximum load the tyre can carry. These are generally valid for speed under 210km/h (130mph). Once you get above these speeds, the load-carrying capacity of tyres decreases and you're in highly technical territory.

For the sake of simplicity, if you know your car weighs 2 tons - 2000kg - then assume an even weight on each wheel. 4 wheels at 2000kg = 500kg per wheel. This is a load index of 84. Then engineer in you should add 10% or more for safety's sake. For this example, I'd probably add 20% for a weight capacity of 600kg - a load index of 90. Generally speaking, the average car tyre is going to have a much higher load index than you'd ever need. It's better to have something that will fail at speeds and stress levels you physically can't achieve, than have something that will fail if you nudge over 60mph with a six pack in the trunk.

Load Index - Kg
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Max Pressure Indicator

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Every tyre has a maximum inflation pressure stamped on the side somewhere. This is the maximum pressure the tyre can safely achieve under load (51PSI in the picture above). It is not the pressure you should inflate them to. Ask your tyre dealer for recommended tyre pressure.

Thread Pattern Denotations

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Symmetrical: consistent across the tyre's face. Both halves of the treadface are the same design.

Asymmetrical: the tread pattern changes across the face of the tyre. These designs normally incorporates larger tread blocks on the outer portion for increased stability during cornering. The smaller inner blocks and greater use of grooves help to disperse water and heat. Asymmetrical tyres tend to also be unidirectional tyres.

Unidirectional: designed to rotate in only one direction, these tyres enhance straight-line acceleration by reducing rolling resistance. They also provide shorter stopping distance. Unidirectional tyres must be dedicated to a specific side of the vehicle, so the information on the sidewall will always include a rotational direction arrow. Make sure the tyres rotate in this direction or you'll get into all sorts of trouble.

Coloured dots and stripes

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When you're looking for new tyres, you'll often see some coloured dots on the tyre sidewall, and bands of colour in the tread. These are all here for a reason, but it's more for the tyre fitter than for your benefit.
The dots on the sidewall typically denote unformity and weight. It's impossible to manufacture a tyre which is perfectly balanced and perfectly manufactured in the belts. As a result, all tyres have a point on the tread which is lighter than the rest of the tyre - a thin spot if you like. It's fractional - you'd never notice it unless you used tyre manufacturing garage equipment to find it, but its there. When the tyre is manufactured, this point is found and a coloured dot is put on the sidewall of the tyre corresponding to the light spot. Typically this is a yellow dot (although some manufacturers use different colours just to confuse us) and is known as the weight mark. Typically the yellow dot should end up aligned to the valve stem on your wheel and tyre combo. This is because you can help minimize the amount of weight needed to balance the tyre and wheel combo by mounting the tyre so that its light point is matched up with the wheel's heavy balance point. Every wheel has a valve stem which cannot be moved so that is considered to be the heavy balance point for the wheel. (Trivia side note : wheels also have light and heavy spots. Typically the heaviest spot on the wheel is found during manufacture and the valve stem is then located diametrically opposite that point to help balance the wheel out).
As well as not being able to manufacture perfectly weighted tyres, it's also nearly impossible to make a tyre which is perfectly circular. By perfectly circular, I mean down to some nauseating number of decimal places. Again, you'd be hard pushed to actually be able to tell that a tyre wasn't round without specialist equipment. Every tyre has a high and a low spot, the difference of which is called radial runout. Using sophisticated computer analysis, tyre manufacturers spin each tyre and look for the 'wobble' in the tyre at certain RPMs. It's all about harmonic frequency (you know - the frequency at which something vibrates, like the Tacoma Narrows bridge collapse). Where the first harmonic curve from the tyre wobble hits its high point, that's where the tyre's high spot is. Manufacturers typically mark this point with a red dot on the tyre sidewall, although again, some tyres have no marks, and others use different colours. This is called the uniformity mark. Correspondingly, most wheel rims are also not 100% circular, and will have a notch or a dimple stamped into the wheel rim somewhere indicating their low point. It makes sense then, that the high point of the tyre should be matched with the low point of the wheel rim to balance out the radial runout.

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Traction
Tyre traction is rated AA, A, B or C, with AA at the top of the scale. This rating is based on the tyre’s ability to stop a car on wet concrete and asphalt. It does not indicate the tyre’s cornering ability.

Of currently available tyres:

3% are rated “AA”
75% are rated “A”
22% are rated “B”
only 1 line of tyres rated “C”

Temperature

The tyre temperature ratings are A, B or C. The rating is a measure of how well the tyre dissipates heat and how well it handles the buildup of heat. The temperature grade applies to a properly inflated tyre that is not overloaded. Underinflation, overloading or excessive speed can lead to more heat buildup. Excessive heat buildup can cause tyres to wear out faster, or could even lead to tyre failure.

Of current available tyres:
27% are rated “A”
59% are rated “B”
11% are rated “C”

Treadware Marking


The treadwear grade describes how long the tire manufacturer expects the tire to last. A Course Monitoring Tire[CMT] (the standard tire that a test tire will be compared to) has a rating of "100". If a manufacturer assigns a treadwear rating of 200 to a new tire, they are indicating that they expect the new tire to have a useful lifespan that is 200% of the life of a CMT.

How long will tyres last

Assuming the CMT lasted for 10 months for your typical driving, then

1) Treadware 300 will give you = 10/100 * 300 = 30 months lifespan

2) Typical CMT lasts for 20,100 kms for typical driving,
then

Treadware 300 will give you = 20,100/100 * 300 = 60,300 mileage

Keeping it Real
Driving style, road condition and differing CMT of various manufacturers will all influence treadware. Manufacturers also tend to be optimistic in stating this figures. Hence the above is meant to be a guideline only (don't let a salesman use these numbers to give you a guarantee). It tends to be more accurate when comparing between models of tyres of the same manufacturer. I would give a 20% reduction to allow for poor Malaysian roads, sprited driving, Malaysian weather etc.

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DOT Codes
and the 6-year shelf life

As part of the DOT code (G in the tyre marking above), there is a tyre manufacture date stamped on the sidewall. Take a look at yours - there will be a three- or four-digit code. This code denotes when the tyre was manufactured, and as a rule-of-thumb, you should never use tyres more than 6 years old. The rubber in tyres degrades over time, irrespective of whether the tyre is being used or not. When you get a tyre change, if you can, see if the tyre place will allow you to inspect the new tyres first. It's not uncommon for these shops to have stuff in stock which is more than 6 years old. The tyre might look brand new, but it will delaminate or have some other failure within weeks of being put on a vehicle.

Reading the code.
The code is pretty simple. The three-digit code was used for tyres manufactured before 2000. So for example 1 7 6 means it was manufactured in the 17th week of 6th year of the decade. In this case it means 1986. For tyres manufactured in the 90's, the same code holds true but there is a little triangle after the DOT code. So for this example, a tyre manufactured in the 17th week of 1996 would have the code 176
After 2000, the code was switched to a 4-digit code. Same rules apply, so for example 3 0 0 3 means the tyre was manufactured in the 30th week of 2003.

Other Markings

Here are some other stuff that may be stated on your tyre.

"Star": Original tyres for BMW
BSW: Black SideWall
FR: Flange Rib - the area above the bead of the tyre that acts as a protection for the outer lip of your alloy wheel against light contact with kerbs etc.
M0: Original tyres for Mercedes-Benz
Made in ...: Country of production
N(number): Original tyres for Porsche.
OWL: Outline White Lettering
RF: Reinforced tyres
SFI, or Inner: Side Facing Inwards; inside of asymmetric tyres.
SFO, or Outer: Side Facing Outwards; outside of asymmetric tyres.
SL: Standard Load; tyre for normal usage and loads
TL: Tubeless
TT: Tube-type, tyre must be used with an inner-tube
TWI: Tread Wear Indicator.
WSW: White SideWall
XL: eXtra Load; tyre for vehicles of heavier standard weights
Arrows: Denotes rotation direction for directional tread.

For European Made Tyres
I saw these markings on my German made 225 Assymetrics

All tyres sold in Europe after July 1997 carry an E-mark. The mark itself is either an upper or lower case "E" followed by a number in a circle or rectangle, followed by a further number.
An "E" (upper case) indicates that the tyre is certified to comply with the dimensional, performance and marking requirements of ECE regulation 30.
An "e" (lower case) indicates that the tyre is certified to comply with the dimensional, performance and marking requirements of Directive 92/33/EEC.
The number in the circle or rectangle denotes the country code of the government that granted the type approval. 11 is the UK. The last number outside the circle or rectangle is the number of the type approval certificate issued for that particular tyre size and type.

Excellent info, making your post as "sticky" so all members can get benefited for it .

excellent - just got myself a new set of tyres .. timely info ...

excellent info!

Thanx bros ... finding out more useful data on various websites. Will keep updating.

Thanks for the info. Learn a lot in this forum.

comprehensive!!! thanks for the info. save lotsa time from searching the net.