Going from 130/80 17 to 120/90 21. What's the difference in overall circumference? I'd like to do the monotube sti drop the front but they're not available for the 14's until October. Trying to get an idea hoe much the ride height will be affected

 

You can't really judge that by the numbers alone, even in the same size, same manufacturer that can vary. Better to go to the manufacturers website and pull up the specs for a particular tire - not sure you'll find circumference but overall diameter should be there, make sure you look at the rim size they take that measurement on as rim size can affect the height of the mounted tire.

 

Oops, going to 120/70 21. Sorry, vodka.

 

Going on a 21x3.5

 

You don't care about the circumference, you care about the radius or 1/2 the overall height/diameter of the tire. That is the number either a + or - change from what you have now, that will show you the ride height change.