When the overdrive button on an automatic gear shift is not depressed, or in the on position, which it where it should be 99% of the time, it is saving you money in gas by lowering the revolutions per minute (RPM) of your engine. Because the transmission is essentially in full control of deciding the optimal gear the car should be in during any particularly everyday kind of driving condition, the overdrive button acts to save on wear and tear in the gear system, which will also save you money. So that does mean you should never turn the overdrive off on an automatic transmission?
Of course not, although the average driver a car or van will rare need to do so and even very few drivers who were suckered into buying an SUV will ever need to do so. Turning off the overdrive button on an automatic gear shift serves to increase the engine’s torque and increases the efficiency of the vehicle when you are either climbing a steep hill or towing something. The only thing is that even if you have an overdrive button to disengage, you could probably accomplish what turning it off will accomplish when climbing a steep grade by shifting to another gear. In reality, the only time you should ever press your overdrive button in on an automatic vehicle is if you are towing something. An added benefit is that it will also help alleviate the damage caused by the increase heat that accompanies towing.