ok just so this doesnt get drawn all out ill explain. The
dealership told me that the transmission is programmed for the 17 inch tires and not the 20's/ I will have to buy a program from gm and have them release it to the dealership. If they will he said. And then they will have to reprogram ours with the 20's. They said the transmission wont shift right and can throw all kinds of codes. Other treads state that traction control can kick on at anytime if you change size of tire. 80 dollars for the program and another 180 to install it. so 260 to reprogram for the 20's in which esculades came out with.
I'd find a new dealership, this one sounds like they're trying to make some money off of you.
Your car's computer doesn't care if you have 20" 17", 18", etc wheels on it as long as the total rotational diameter is the same or close to the same. The only ill effects you'd see with a slightly undersized or oversized tire is speedometer error or slight fuel economy differences if you under or oversize.
You're not going to affect the driveability unless you do something drastic like put way undersized or oversized tires on it or different diameter tires between front and rear or side to side.
Needing special programming is suspect as well. I have the same Tech II that the dealership uses and I can select the tire size right from the tool without any special programming or interface with GM TIS. It has built in the various tire/wheel sizes that the Escalade came with no, 'special' program needed.