Newbie here....I had a 92 Suzuki then I totaled it. Now I have 02 G. Vitara. Question: currently on the 02 are 16" 235/60 all season  tires but I have the rims/tires from the 92 which are 15" 215/75 BFG ATs. Can I put them on the 02? It would seem to me that I could as I measured from outside edges of both tires and the 15" are 26". I've read about plus sizing but I couldn't find info about
"downsizing." 

Probably not - the 15" rims will most likely foul the front brakes on the GV - although it may depend on exactly where you are and what engine is fitted to the GV, some models had smaller brakes.

If they clear the rotors you can fit them, but you can expect to perhaps burn a bit more fuel - the 235/60R16 are a 27" tire so slightly taller than the 215/75R15s, the speedo will read over, the car will accellerate faster, all of which can be fixed by going up to a larger tire at the next tire change.

I'm pretty sure that a 15" rim will not fit the GV.  I seem to recall that they have larger calipers than the 4cyl and 6cyl Gen2 Vitara and Trackers made in Canada that came with 15" rims.

Yup - depending on where you are the engines and brakes can be different - some countries got a 3 door GV with a 1.6 engine, those have smaller brakes - I have a 5 door GV with a 2.0 I4, and that's got the larger brakes.