I have a set of stock summer tires from a Toyota Yaris will they fit the stock rims of a 2004 chevy cavalier?
you need to check the speed rating and load rating of the tyre, normally indicated by something like 91W or 91W...95W etc. If the cars are drastically different in terms of speed and weight and are also an important factor.
In the event of an accident if you dont have the correctly rated tyres (as specified by the manufacturer) insurers will use that as a get out clause.
Normally on tyres you can go up/down 10mm from the specified tyre width without too much of a problem - i.e. if a manufacturer says "215" then in theory you could go to 205 or 225 width tyres, but I wouldnt push it further than that.
Compare tire sizes. If the tires you have on your car right now match the recommended factory tires size, and the Yaris has this same size, it should be fine. Mostly tire size and rating is what you compare, assuming stock rims of course.