I bought 5 tires for my 940, size 185/65/15R. I didn't realize the spare was one of those temporary tires.The tire size was 155/R15/90. I went ahead and removed the dry rotted spare from the rim.

Great! I thought. I should have only bought 4 tires and found a different size for the spare. But then I decided to put one of the new tires on the narrower rim. It appears to fit fine. It also fits fine in the spare tire compartment. I searched the web, but I can not find anyone who has done this. They run out and buy a new rim to install a full size tire. Now I realize that the spare rim is an inch or so narrower than all my other tires, but has anyone else ever done this? Why or why not is it okay to do this?

 

a 185/65r15 is rated for mounting on a 5-6.5" wide rim, and its dimensions are given for a nominal 5.5" wide rim.

I can't find the dimensions for a 155/90R15, but a 155R15 is rated for a 4-5" rim.

btw, my 940 had 195/60R15 stock, although I replaced it with 205/55r16 via Hydra wheels.... ah, mine was a wagon, yours is probably a sedan. the 195/60 has a bit wider tread.

 

Thanks! I think I measure the 155/90R15 rim at 5.5" (outside width, so inside width is right around 5"). I guess my converted spare should be okay.