The reason you should not use overdrive all the time is to keep your transmission cool. Overdrive is a gear used to help your gas mileage when "cruising", not while "working" your engine. When going uphill, your transmission is downshifting to gain RPM's or power. If you are towing in overdrive, you are causing your transmission to shift under a load, in-and-out of the overdrive gear. This causes heat, just like your engine heats up under a load going uphill. When your transmission parts get hot they are softer and can break more easily, especially under a load when towing.

If you tow a lot, it is recommended you install a transmission cooler to your tow vehicle. This is like a small radiator that is installed near the front of your vehicle. It circulates the transmission fluid through the small radiator and dissipates the heat better than before. That will greatly help the heat issues.

If you’re towing a large boat or trailer, it won't matter how cool your transmission is if it is not strong enough to tow. In other words... don't tow a 23' Parker with your dad's Toyota Tecoma. You need the right tool for the job! As far as using overdrive going downhill... you take it out of overdrive to increase the RPM's of your engine. This will usually cause you to slow down so you don't have to ride your brakes all the way down the hill. Therefore maintaining your braking ability (no hot brakes) and less wear on the brake parts.

I hope this helps a little... or do I just have too much time on my hands?!!

Terry G.