If I switch out my stock 255/40 R19 and 275/40 R19's to 305/30 R19's and 315/30 R19's. How much wheel well gap am I going to see without adjusting springs?
Using the tool above, the front tire will be -.8" / 20mm (-3%) less in diameter (255/40-to-305/30).
The rear, - 1.3" / 33 mm (-4.7%) less in diameter (275/40-to-315/30).
A general rule of thumb is to stick within +/- 3% in circumference/diameter of the existing tire setup. People can and do go beyond 3%, but the risks are impacts to shift points, ABS brake points and speedo.
Edit: the 305/315 setup, if it were to happen, could cause "poke" outside the body lines in the front. A 315 rear I'd think would be OK, given enough offset. The fronts (305) though, you'd need to budget for the fender lips, the small extensions to compensate for the rim and tire extending past the body.