I just went from 235/55/17 to 255/45/18 tires. Is it necessary to change these parameters in the tuner? And how would I do it, just change it then re-upload the tune?

 

I don't think it absolutely necessary but it would not hurt...

 

Wheel 235 55 17
Overall Diameter: 27.17 in 690.11mm

Wheel 255 45 18
Overall Diameter: 27.03 in 686.56 mm

Not necessary diameter is quite the same

 

The XCAL-2 unit itself can only adjust the wheel size in increments of 5 revs/mile. Your stock revs/mile is 742, your new one is 746. You could put 745 in the XCAL which would put you very close. But even if you leave it alone you're only going to see a 0.3mph difference on your speedometer at 60.

However, whatever shop did the tune for you (Brennspeed in this case) can set it exactly with their SCT software, email you the new tune, and then you can upload it.

 

I didn't bother with it on my change to 255/45 and again didn't bother with it when i went to 285/40.
Too close to each other to matter