This can be figured out with a bit of math. The difference between 5.25 and 3 is 2.25. So it will automatically stick that much farther out. If you have stock rims they are probably 7 or 8 inches wide. Lets split the difference and do 7.5. Going to 12.5 inches the difference is 5 inches. Split that in half since the other half of that will be going under the fender. This leaves you with an additional 2.5 inches wider than stock. Add those up and you are 4.75 inches wider than stock at the outside edge of the wheel. Technically I do not guarantee these numbers, but that is what I would expect to see.
Check out my build thread!