how sensitive is the TCS to tire size
I'm planning to put some (in the rear) 18X10's with 295/35-18 and that comes to 22.06496 inchs
and the stock are 22.08858 inchs 235/55-17
I want my fronts to be 18's but close to stock
so how close or far apart can they be in size without messing up TCS

 

I checked and if I go with 295/45-18 in the rear then the diameter is 23.226378 inch
and I can go with 245/55-18 in the front ant that has a diameter of 23.3501181 so only about a tenth of an inch
but would this be enough of a change to need to reprogram

 

I don't know the exact threshold off the top of my head (for some reason 4% comes to mind) but I can tell you that going from stock 235/55/17 Pirelli's to 255/40/17 BFG KDW was enough to cause my TCS to kick in w/o adjusting the rev/mile.

 

did you change just the back tires or all four
I did the math and stock 17's take 913 revolutions for one mile
and the 295/45-18's take 868 revolutions
and the 245/55-18's take 865 revolutions
so to I will need to correct the speedometer
but can the TCS be corrected for the slight diff

 

I changed all four tires. BTW, you might be better off using the mfg tech spec for the specific tire you are using vs a tire calculation.
- eg.

My calculations come out different...
For my BFGs (245/45/17) I get780 rev/mi(roughly) and using a couple diffonline calculators I get about the same.
But if I go by BFGs tech spec for the tire I get 810....which is what I have used when I reprogram my 'puter.

For the stock 17s (thats 235/55/17 on my '05GT) I get around 745 rev/mi. I can't find a tech spec for the PZNs anymore but I seem to recall it was around 780 rev/mi.