I have a 1990 240 with the old 84 turbo wheels. Currently it has 195/60/R15 tires. How wide of a tire can I put on these wheels? What about 215/50/R15?

That's only a 6 inch wide wheel. While others may disagree I wouldn't go any
wider than a 205 tire width on those wheels. A good performance tire size
is 205/55-15. Personally, on the road I don't like any lower profile than 55
on a 240. The reason being that it's a 70s suspension design and it really isn't
designed to handle the extra dampening loads that an extremely low profile tire
requires. This is my opinion from my experience. Bilsteins, Koni, and shocks
of this quality then maybe a 50 series. But you really sacrifice any bit of ride
quality. My setup is IPD springs, Bilsteins, IPD sway bars. In the summer I use
the 205/55-15 tires and in the winter I goto a 205/60 so the tires provide a
bit more dampening. This is in NJ. If you can get yourself some leftover Bridgestone RE71 tires from the tirerack in the 205/55 size you'll have tires
that stick like glue. The quality and stickiness of the tires is more important than how wide they are. Hope this helps you.
Dave 82 242ti