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

Once you establish that mental image, any confusion disappears. Overdrive means that you are using the largest gear in your transmission to spin 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’re driving at a higher gear, it would stand to reason that you’d get better fuel economy because you’re being driven by a larger gear that spins less. Overdrive lowers engine RPM, which saves gas and reduces engine wear. Overdrive is best for long, flat stretches of road. I always go into overdrive when I know the road ahead is not hilly and I will be able to drive without interruption at high speed on the freeway, and I am not towing or hauling an extra heavy load.

You can also just leave the overdrive switch “on” and this will happen automatically. I recommend overdrive for speeds of 60 MPH and above. When should you NOT use it? Never use overdrive when you are driving in hilly areas, towing something, or traveling very slowly. In fact, you could damage your transmission if you do. Remember, a large 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 high, steady speed, and saving you gas while doing it.

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

You will then 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 drive the wheels and the RV would vibrate and shake. You should never use overdrive when driving in these more stressful conditions. I leave the overdrive off and manually flip the switch to “on” when traveling long stretches of flat road.