It is more economical to drive with the OD light on. Only take it out of OD if you are on hills and the car is shifting a lot

This in an OD OFF warning light. If the OD light is on then the car is NOT in OD!
It is considerably more economical to drive with the light off. You can switch it on and off while highway cruising and see the RPM drop. Leave the OD on all the time. (Keep the light OFF) for the best mileage.