I'm not 100% sure, but it appears that I'm getting better gas mileage without Overdrive than with it. I'll know today or tomorrow when I refuel.

If this is the case is it possible to change the O/D switch to be normally off and selectable on, instead of the normally on and selectable off?

 

I don't think you can. My understanding is that it is a momentary switch that sends a signal to the ECU to shut off the OD if it is on, or turn it off if it is on.

Now, if you wanted to get real funky, you could wire up a small delay timer that closes the contact say, 60 seconds after key-on.