x,
145 is the width,in milimeters of the body (not the tread) of the tire at its widest point when mounted on a wheel which is the proper size (width) for that tyre. 70 is a measurment expressed as a percentage of the first number. This second number expresses the height of any given section of the tire from the bead (where it mounts to the wheel) to the crown, or tread face (the part that contacts the road). In this case, that would be 70% of 145 milimeters, or 101,5 milimeters. The R refers to the fact that this is a radial tire. Anymore this is somewhat supurfluous, as almost all tires with these other dimensional details are radials, or as they say on your side of the pond, "metrics". You will see other letters paired with the R marking, such as T or Z. These refer to the duty and speed rating of the tire. The 12 refers to the diameter of the tire mounting surface of the wheel, in inches. As you can see, with this information you can determine all of the critical dimensional data about any tire thus labeled.....almost. There are slight differences from one manufacturer to another. Most of the major tire makers maintain a website with dimensional data shown on a table. Many even show revolutions per mile, which may be useful to those looking to economize or for ground clearance or performance issues on modified vehicles.
I'll be darned if I know very precicely what tire, excuse me, "tyre" size is original on your car for your home market. Perhaps you could contact a local club. Each club has one or two guys who seem to have instant recall of every little and big detail for their favorite machine. If I took my best guess, based on the cars and data we have here, it would still be just that..a guess. Cheers.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon