Looking at a newer LS, a 12 vs my 07. The 12 has optional 19 inch wheels. I have 18's on the 07. I have 18 inch wheels and winter tires. What would happen if I swapped the 19's for the 18's in the winter? Do they calibrate the speedometer when they put the 19's on?

 

There's no calibrating the speedometer unless you are changing the total diameter of wheel+tire, and you generally don't ever want to do that. 18 inch wheels have thicker sidewall tires, 19 inch wheels have thinner sidewall tires, resulting in the same (or very nearly the same) total diameter. I change out 19" summers for 18" winters on my BMW, and the only things that need to be "recalibrated" are the TPMS sensors, which is just a setting in the car's computer.

 

+1 with roger

try this and see how much the difference is.
https://www.tacomaworld.com/tirecalc