Can anyone explain how or why I gain 2-10's in the 1/4 when I have the Overdrive on.

It defies logic... well my logic.

TCS IS off...
All things being equal...
My car is 2-10's faster with overdrive on.

 

I discovered the same thing just yesterday. I switched over to a 3.73 in the rear end, and noticed that my speed was down and the car seemed to be quicker at first but then just "ran out of gear". I left the OD on, noticed it shifted again just before the traps, and my speed and time were better.

 

I like the "ran out of gear" description. That's what I am talking about.