Judging from recent posts, some people on here need to go through the basics. I've knocked up this quick guide to cover the main points 

Rolling circumference

The rolling circumference of your wheel AND tyre is defined as the outer circumference of the tyre when fitted and inflated. This has an influence on the way your car will perform, in the same way as being in a certain gear does.

A 205/55/15 tyre has a rolling circumference of 1850 mm. That means if you rolled it by hand, through one complete revolution... it would travel 1850 mm

A 225/40/18 tyre has a rolling circumference of 1943 mm. That means if you rolled it by hand, through one complete revolution... it would travel 1943 mm

Now imagine a Calibra moving steadily down a road. The gear it is in, and the engine speed just happen to mean the wheel hubs are turning at 1,000rpm.
1) With the 15's fitted, that would mean the car moves 1850mm every revolution. SO, at 1,000 wheel revolutions per minute, the car moves 1,850 metres (yes I converted that to metres) per minute
2) With the 18's fitted, that would mean the car moves 1943mm every revolution. SO, at 1,000 wheel revolutions per minute, the car moves 1,943 metres per minute.

Therefore bigger rolling circumference = higher speed at any given RPM. The flip side is, acceleration will be slower as the larger circumference is like being in a slightly higher gear, all the time.

What affect does this have on your speedometer? Well compared to a fictional "standard" tyre tyre size of 185/65 R15, it would mean that on the other 15's your speedo would read 97.6km/h @ a true 100km/h whilst the larger 18's would mean your speedo reads 105.1km/h @ a true 100km/h.

Tyre markings

Here is how to identify your tyre


1) "205" - this is the width across the tyre in millimetres.
2) "45" - the aspect ratio in % (the height of the sidewall divided by the tyre's width)
3) "R" - Radial construction.
4) "17" - the diameter of wheel rim the tyre is intended for
5) "84" - the load capacity index.
6) The speed index (see below)






Tyres for sale in the European Community must carry an E -Mark in accordance with ECE Reg 30 - eg E4 027550.

PCD

Pitch circle diameter, or PCD, is a way of defining the bolt pattern of a wheel.





All 4 stud Calibras are 4x100
All 5 stud Calibras are 5x110

Note some other manufacturers share PCD values (ie VW 4-stud is also 4x100) but the offsets are often not good for Vauxhall... see next sub-heading

Offset
Offset in laymans terms is "how far the wheel sticks in or out". Technically it's defined as the positive or negative difference in distance between the vertical plane of the wheel mounting face and the centreline of the wheel.

Here is a diagram showing the three styles of offset


All 4 and 5 stud Calibras use ET49 from the factory. It is important when choosing replacement wheels to get as close to this as possible so as to reduce the chances of your wheels scrubbing. This is why a set of 4x100 wheels for a Volkswagen will fit a Calibra, but chances are the offset will be wrong.

Here is a real life example:
ET38, stuck out a lot, scrubbed an awful lot, even on empty car with low profile tyres


ET49, sat perfectly, does not scrub at all even on higher profile tyres with a car full


Centre bore
This is the diameter of the hole in the centre of the wheel-to-hub mating face. Calibra centre bore is 56.6mm. Any bigger and you need spiggot rings to centre the wheel on the hub. Yet another reason not to buy multifit wheels.

Other considerations when buying wheels
Weight has to be the biggest factor in buying a wheel for a performance car. They are a huge source of unsprung mass which has a bearing on ride quality and grip. Wheels like Compomotive and Team Dynamics pro race can offer strength and a very low weight compared to the super heavy "bling" wheels.

Any questions feel free to ask! 

Further reading:
Tyre calculator: http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC%20calculator.htm

Nice one Chav

so if the tyres ware on the inside like there is no tomorrow then that is positive offset?? cuz that is what mine are doing!

How much the wheels lean in/out at the top or bottom is the camber (this is what causes uneven tyre wear that you have)

An example is when you lower a Calibra, the rear wheels have too much camber (ie the top edges of the wheels lean in). This is nothing to do with offset.

There is also something called toe, but forget that for now

They are different to offset

is there any way to fix this?

Assuming this is your rear tyres - fit some camber correcting bushes. A short term fix is to rotate your tyres.

mine is the front that is doing this! is it still the same thing?? (bushes)

Not sure on the fronts mate, is it lowered a lot?

no not really the wheels where on my other car for the most of the time..but that was only lowerd 30mm..but the car i have now is only lowerd i think about 20mm

So i'm alright just driving it then??... im farely sure my tyres rotate as i drive.. could someone clarify this for me?

you funny git!! lol.....has to be the funnyest thing i have heard you say!!!