Like the title says what are the biggest wheels you can fit on a gen 1 brat?

With the stock 13" steelies maybe just 185/70's but with pugs maybe a 205/65/14 or with the 15's.

you can run 15inch rims, but its all on tire size..as jerry stated..

Make sure that whatever you put on that you test it before you buy or install it. The Gen 1 Brats have a different backspacing. I discovered that when I tried to put stock subie steelies from a 88 XT on my 78 Brat. The backspacing was wrong and the rear wheels did not fit. Well they fit, They just would not roll. Some of the guys on here should be able to give you exact specs as to what will clear in the back.

also your springs will make a heck of a difference, if there fresh you will be able to get away with more, and if there weak, then the tires might rub.

I have 15's with 55's but they barely rub gonna try 50's