All valuable input and information but remember the purpose of an "overdrive" gear.
It is not to increase top speed. Of course it will decrease top speed as it reduces the torque at the wheel(s) and drag will triumph.
It is there to "relax" the engine under normal "cruising" conditions.
It is not meant to be used when maximum speed or overtaking accelleration is needed.
On any older vehicle you would achieve maximum speed in "top" gear. On most modern vehicles you will almost certainly achieve a higher top speed using a lower gear.
On the subject of fuel consumption, of course , if the engine is labouring into a head wind or uphill, fuel consumption will be worse in an "overdrive" situation.
Again, that is not when you should use an overdrive gear. Also, please remember that overdrive is not necessarily a physical ratio.
It is a gearing situation in which maximum speed is achieved in a lower gear than the highest one.
Any overall ratio that reduces maximum speed is "effectively" an overdrive if not a physical one.
Macka
Exactly my point. But....I still want the extra 20hp to go with it