Hey Dave,
I have fitted an XR6 diff and speedo cog to my car. I didn't need to change anything on my
ecu when I got mine done. This was all before the s/c and any tuning. Drove it fine as it is. Still using the stock
ecu as well. The
Ecu is a learning computer as you know anyway, so it will get used to your driving style and cater for any changes. I normally drive around without the econ button on anyway. Seems to hold revs higher before changing anyway.
Not sure if that helps or not.