The stock/factory wheels are 13 x 5-1/2" (4 x 110 x 25 offset). Can you mount 205x50's on these wheels or do I have to go with 7" rims?

 

you can physically do get 205, (I dunno about 50, usually 60+) but shoudl you? Most say you actually get a lot better cornering from a narrower tire because the sidwall has a lot less bulge.

Look up the specs for the specific tire you are wanting to run and see if the manufacturer says you can. That would be my personal reccomendation.

 

Correction. The Toyo' RA-1 are 205/60's.

 

The GSL-SE comes stock with 14x5.5" wheels and 205/60 tires. You should be able to run your combo without difficulty. Just look around to make sure they aren't going to rub on anything.

 

I'm planing on running ITA (SCCA) and for the '79 FB need to stay with 13".

 

I wasn't saying you should go with 14s, only that the SE came with 5.5" wide wheels and 205/60 tires. Mazda already put tires that wide on rims that narrow. Sorry for the confusion.