http://www.carbibles.com/tyre_bible.html

scroll down a bit, theres calculators to tell you the range of tire sizes you can fit on a rim, and to calculate rolling radius (which is what you wanna keep the same as stock)

 

which one do i need to use? the one that says "a tyre size calculator". what is an acceptable difference?

i put in that i have a 225/45/17 and want to go to a 235/35/19 it tells me that the difference in circumference is 40.21mm but that doesn't really mean a lot to me. what exactly am i looking for? sorry about all the questions but this is neew to me. thanks.