The easiest way to think of overdrive is to associate the word “overdrive” with the phrase “really great gear.”

Once you establish that mental image, any confusion disappears. Overdrive means that you are using the highest gear in your transmission to turn the wheels, at high speed. Because a large gear has more metal teeth than a smaller gear, the large gear has to turn less (less RPM) to maintain the same speed as a smaller gear.

So when you drive at a higher speed it stands to reason that you will get better fuel economy because you are driven by a larger gear that turns less. Overdrive reduces engine RPM and that saves gas and engine wear. Overdrive is best for long, flat stretches of highway. I always go into overdrive when I know the road ahead is not hilly and I will be able to drive smoothly at high highway speeds, and I am not towing or carrying an extra heavy load.

You can also simply leave the overdrive switch “on” and this will happen automatically. I recommend overdrive for speeds of 60 MPH or more. When should you NOT use it? Never use overdrive when driving in mountainous areas, towing something, or traveling very slowly. In fact, you could damage your transmission if you do. Remember, a high gear has to turn less to maintain the same speed as a smaller gear. So at higher speeds, the big gear is doing its job, effortlessly pushing you along a long, flat road at a constant high speed, saving you gas as you do so.

But when you’re traveling at slower speeds or stopping and starting or going up and down hills or towing a car, it’s less of a high-speed point-and-shoot trip and more about having quick power to accelerate or being able to navigate. through hills or mountains, or being able to tow a heavy load safely. At a lower speed, the large gear turns less and less and loses its efficiency over the smaller gear.

Then you would need to downshift to generate enough RPM to provide power to the wheels. If you didn’t downshift, you would quickly feel the higher gear struggling to power the wheels and the RV would vibrate and wobble. You should never use overdrive when driving in these most stressful conditions. I leave the overdrive off and manually flip the switch to “on” when riding long stretches of flat road.