ITHINK I read somewhere on this board some time back that another 05 SSR owner found out that the dealer's Tech II unit could alter the gear ratio in the PCM to 4.10, but not 4.56.
If true, the reason is PROBABLY that 4.10 is still offered as a factory option by GM for SUVs ad pickup trucks in the GM line-up, for trailer towing applications (they can't get the required GWCWR without the high ratio), while 4.56 is NOT currently available from GM.
So, ask your Chevy dealer.
This is a question we SHOULD be able to ask GM, but they apparently prefer ONE way communication - their advertising to us - rather than 2-way.
That's why I don't know for sure if my theory is correct.
Jim G