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Good morning and thank you very much for your question.
Turning off the overdrive on your automatic transmission car has the same effect as changing up a gear with a manual transmission car when coasting down hill. The lower the gear, the greater the engine braking. This is called engine braking.
With automatic gearboxes, the gears will not always change down by themselves, so you need to either use the gear lever to change down, or turn off the overdrive.
The main reason you'd want to employ engine braking is to descend a steep hill. In such a situation, riding the brakes might overheat them and cause them to fade. It is absolutely fine to turn off the overdrive when coasting down hill, and will not cause any damage to your transmission.
I hope this answers your question, but if you need any further help, please reply to this answer.
Best regards, Gary.
Thank you for your reply.
The overdrive in on when the light is illuminated.
Sorry, you need to turn the overdrive off which will increase the engine speed and increase engine breaking. This will have the same effect as changing from fifth to fourth in a manual car.
I hope this helps.
Yes this is the opposite to what I would expect in the UK. Perhaps things are different in your part of the world. Turning the overdrive on when cruising will lower the engine speed.
An increase in engine speed will increase the engine breaking effect. So to slow your car down when driving down hill, you need the engine speed at it's higher speed setting.