I was using an ancient P195R14 as a spare, but even it was 3/4 inches too wide. I put pieces of 3/4 ply on the trunk floor on either side so the board would lay flat and not crack (this was in a cuda fastback, where the trunk is really small). 185 is the width that would lay flat, but I don't think anybody makes a tall enough 185-14 -- it would need to be an 80 series. Maybe Coker would have something in a vintage size, but then you wouldn't want to mix bias-plies with radials, either. Seriously, I've looked at it, and it's hopeless -- no current 14in radial tire is narrow enough at 25.5in diameter. When my fastback is back on the road, I'm just going to get a matching rim with a full-size tire, and put it behind the seat under the glass, like a European rally racer. I'll put a toolbox in the useless well.
Some people have found those narrow mini-spares in a usable diameter, but you need to be running Large Bolt Pattern to use any of them.
Hmm, it just occurred to me that my convertible's spare doesn't bulge hardly at all. I wonder what size it is? It is probably 25-30 years old, though. Just went and looked -- it is a Goodyear Vector 175/75R-14. The diameter is really small -- maybe 24.5in. But even the 175 width bulges the floor up maybe 3/8 inch. This is on a 4.5in wide rim -- on a 5.5 the sidewall might be a little flatter.
Thinking about this makes me wonder if there would be a market for a retrofit trunk floor/fuel tank combo with wider and deeper tire well, for contemporary size tires...
, but since my Duster is my daily it's nice to have a spare available. I've since gotten rid of the bottle jack and picked up a scissor jack at the local picknpull. It's a little better suited for the intended use, and a lot lighter. 
