Well, i was just experimenting with O/D on and off, and it seems that when i turn O/D OFF, i get poorer mileage. I've heard that overdrive should really only been used when trying to go up mountains / down them, but ive never heard of turning it off causing both speed increases and a drop in mileage. I don't get it, maybe because i'm driving more quickly with it off will cause the poor mileage; but i don't understand how i'm going faster with it off in the first place.

 

When you turn O/D off, you are telling the automatic transmission not to shift into the highest gear (4th/overdrive)

That means you use only your 3 lowest gears, when at highway speeds you'll be revving at a much higher RPM than you need to, making the engine work harder.

To clear things up: O/D should be ON most of the time. The only time you would want to turn O/D off is when you do NOT want the car to go into 4th gear/OD.

An appropriate time to switch O/D off might be when you are climbing several steep hills in a row driving in the mountains for example. If you are carrying a full load of passengers with the air conditioning on and the car seems sluggish, thats also a good time to turn of O/D.

Generally, in around town driving overdrive is going to save you a little fuel with little penalty so I (and Toyota) recommend leaving O/D on the majority of time.

Hope that helps.

 

Exactly what i was looking for, thanks. Lock It Up boyz