I've noticed a lot of people asking questions on here about tire sizing who obviously don't understand what they mean. So, I thought I'd be a good forum denizen and put my knowledge and writing skills to work.
Tire Sizing Decoded
For this example I'm going to use a tire sized as follows: P245/45ZR18
P = Passenger Car tire (whereas LT is for light trucks like pickups and SUVs)
245 - tread width in millimetres. In other words, the width of the tire where it contacts the road.
45 = aspect ratio of tire's sidewall to width (as a percentage). So, in this example, the sidewall would be 245mm x .45 = 110.25mm.
Z - speed rating (i.e. how fast the tire is rated to go). A 'Z' rating happens to be the highest for passenger cars, able to travel at 186mps (300 km/h). However, most passenger cars make due with far less capable tires (what normal car can go that fast anyway?)
R - radial. This harkens from before radials were the norm, back in the days of the infamous bias-ply tires (before my time though).
18 - wheel/rim size in inches. Enough said.
Calculating Tire Diameter
This following may seem a bit complex to some, which is why I've also included a link to a javascript calculator made for this purpose.
Tire Diameter (mm) = (((tread width x aspect ratio) x 2) + (wheel size x 25))
Note: The above formula assumes all values are in their original formats (i.e. mm or inches). To convert millimetres back into inches just divide by 25.
Links:
Miata.net
Why Does Diameter Matter?
Since the circumference of a tire is directly proportionate to its diameter (you do remember grade 5 math don't you?), the one will change with the other.
The speedometer (and odometer/trip-meter) in a car are calibrated based on the size of tire the car came with. If you put on a tire that is bigger or smaller than the original, your speedometer will no longer be accurate (although most aren't too accurate anyway - Car & Driver did a test on that a few years back - if I recall correctly, japs were most accurate, euros were least accurate, and domestics were somewhere in the middle).
If you put a larger tire on your car, it will take less revolutions of that tire (versus the orginal smaller tire) to move a given distance. Thus, your speedometer will tell you you're going slower than you really are, making it likely you'll encounter the local constabulary sooner or later.. not a good thing. Putting on a smaller tire will have the opposite effect, making your speedometer show you're travelling faster than you really are... at least you can't get a ticket for that.
As well, the ECUs in some newer cars don't like it when the tire size is changed, and will throw a big hairy fit as a result. So, in short, it's best to keep the diameter of your tires the same as orginal equipment.
What is a Plus-1?
This is basically used to describe the act of increasing the size car's wheels/rims by one-inch while keeping the overall diameter of the tires fairly constant. As a rule of thumb, this is done by increasing the tread width by 10mm (i.e. 255 instead of 245) and decreasing the aspect ratio by 10% (i.e. 35 instead of 45), and increasing the wheel/rim size by one inch. It's quite difficult to get the diameters to match exactly, so generally you just have to make due with the closest match.
Hopefully this information has been useful to you. I'm sure other forum members will insert their valuable information after, so go read that too...