Mrs Google wasn't much help trying to figure it out.
Which do I need - 195/65/15 or 205/55/16? I figure I am best to go for the 195/65/15 as it will be narrower for the well, and it is only a "get you home" tyre. 195*65% = 127 whereas 225*55% is 124 which is quite reasonable.
A quick lesson:
205/55 R 16:
205 is the nominal width of the tread in mm. Nominal because it varies a bit between brands.
55 is the aspect ratio - the height of the sidewall as a percentage of the tread width.
R is the speed rating, but it's not the whole story as there will be a W,H or V marked elsewhere that goes with it. You
must not fit a lower speed rated tyre than the car is capable of.
16 is the diameter of the wheel in inches.
7J 15 relates to the wheel(an Omega one is more likely to be 6J 15), which means 7inches wide and 15 inch diameter. You might also see something like 38ET quoted; that is the offset, and it's where the mounting face of the wheel compared to one edge(I can't remember which, but it's probably the inner as that has to clear the suspension). You can fit a very wide range of tyre widths and sizes to this width wheel.
You are not supposed to mix tyre sizes on the same axle(it's illegal), but a spacesaver spare does just that by juggling tyre width/aspect ratio/diameter to give what is basically the same overall diameter(rolling radius). It is still considered a spacesaver for temporary use even if it's a normal tyre size or wheel and will have a sticker that says so. Which makes a 195/65 15" tyre on a steel wheel, that was fitted to all four corners of original poverty spec Omegas, a spacesaver on all others. That size was very common on Granadas, Mercs, BMWs and other largish cars in the late eighties and through the nineties until we were all brainwashed into wanting massive diameter low profiles for some reason.
What this means is that ideally you want a spare wheel and tyre that is exactly the same size, width, diameter, speed rating as the ones on the car. You can do that without any problems on an estate, as the wheel well takes all of the stock size wheels and tyres. This isn't necessarily true of saloons but if I had one on 16s, I'd be looking for one of those and living with the tighter fit and not having the cover on it.