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I have the above-described vehicle and I have asked this question to several mechanics in the 4 years I've owned it. But non of the mechanics has been able to explain it to me in simple english. The question is :

What is the purpose of the "over-drive" and when should one use it or not use it?

Would appreciate feedback.

Overdrive (OD) is used as a way of allowing a vehicle to be driven at reasonably high speeds with reduced engine speed, with the ultimate goal of providing improved fuel consumption.

The traditional transmission was designed so that the top gear was a direct one-to-one drive between the engine and transmission. Adding an overdrive gear means the final drive side of the transmission is turning faster than the engine hence the word ‘overdrive’.

In some transmissions a overdrive button was added to the side of the gear selector so the driver could switch the overdrive 'on' or 'off' as required. This has created great debate over the years as to when the overdrive should be engaged/disengaged.

Some say OD should only be used at highway speeds, while others say around town is fine in certain conditions.

The reality is the transmission will only change into any gear when certain conditions are met so leaving the OD on is not really a huge issue.

Disengaging overdrive when descending long and windy terrain can also be beneficial as it provides some engine braking assistance.

In the more modern electronic transmissions, this is all taken care of ‘automatically’ based on several different input signals received by the on-board engine and transmission control modules such as vehicle speed and throttle position.