Also called 'section width'. This is the width of the tyre in millimetres (mm). So if a tyre is 235/40/18, the tyre is 235mm wide (9.25 inches/23.5cm).
Also called 'sidewall aspect ratio'. This is the measurement of the side wall of the tyre. It is not a cm/inches measurement, but a percentage of the tyre width.
For example, a tyre size for a 195/50/15 tyre could be read as '195mm wide/50% of 195mm = 97mm sidewall height/15inches wheel size'. So the sidewall height is actually 97mm (3.8inches/9.7cm).
TYRE SIZE (WHEEL SIZE):
This is the size of wheel the tyre should fit onto. For example, a tyre size of 235/40/18 should be suited to an 18inch wheel.
WHEEL SIZE (DIAMETER):
This is the usual term for describing wheel sizes. An 18 inch wheel has a diameter of 18inches.
When looking to fit new wheels you need to know how wide they are. This is usually stamped on the rear of the alloy, followed by a letter. Usually, it is something like '8J', where the '8' means '8inches wide'.
Also called the 'PCD' (pitch circle diameter). This is the number of wheel bolts per wheel and the pattern they form on your wheel and hub.
For example, your average Audi A4 has a PCD of 5x112. This means 5 bolts per wheel, set out in a circle measuring 112mm.
This is the distance in millimetres from the wheel hub on the car to the centre of the wheel.
In simple terms, a wheel with an offset of '35' will stick out more from the arches than one with an offset of '55' (assuming all other wheel measurements are the same/wheel width etc). Some people use 'spacers' that sit between the wheel and the hub to make their wheels stick out more. Adding a 10mm spacer would reduce the offset by 10mm.
**PLEASE ADD YOUR 'TRIED AND TESTED' WHEEL AND TYRE COMBINATIONS BELOW TO GIVE OTHERS AN IDEA OF WHAT WILL AND WON'T FIT. ALSO ADD YOUR VEHICLE MODEL AND ANY RELEVANT MODIFICATIONS (LOWERING SPRINGS ETC)**
Also, links below (thanks to myturbodiesel.com) for OEM Audi wheel sizes for 5x100 and 5x112 wheels:
16" and 17" Audi stock wheel gallery database5x100
18" VW and Audi wheel picture gallery and database 5x100 bolt pattern
15" and 16" audi OEM wheel pics 5x112 bolt pattern
17" Audi 5x112 wheel gallery
18" Audi 5x112 wheels
19 20" 21" VW and Audi wheels 5x112