Roughly, you can figure out what tire width you need by putting things of different dimensions in the spare well until the pad bows upwards. Place this item where the sidewall normally rests on the bottom of the well.

Then take the measurement in inches and multiply by 25.4, then round down. That will give you the three digit code in the tire. For example, let's say you measure 7.5 inches. 7.5 x 25.4 = 190.5. The maximum tire size less than or equal to this is 190. Your tires are 5mm wider (195). So, to fit in the well you would be looking for a P190Rxx/15 tire. When I did my VW, I called Tirerack at this point and they found a steel wheel/tire in stock meeting my requirement. I told them what I was doing, and while they were not thrilled, they understood and helped me out. You need to know how deep the hole is though.

Note this is just an example.

Good luck. Don '95 854T 186K '05 V702.5T 45K