@simonkabz & @kanda; you are both spot on.
The greatest factor in fuel consumption is not the size of the engine,nor the transmission system (auto/manual/4wd/2fwd/2rwd Awd etc)! It is in the way you drive!! Consider this: they who delivered my Mazda Familia 4wd AT from the port to Nairobi consumed 60 litres of fuel on the journey. When I go on holiday to the coast with the car nearly fully laden,I consume about 40 litres on the returm journey.
Same car,same trip and one driver takes 50% more fuel. And dont say that he cheated me. No,he's my buddy and I know how he drive....he imagines he is Shekar Mehta!!!
On this overdrive thing: it is confusing....in some cars it is labelled OD and in others it is labelled LOCK.
If your car is labelled OD:
Engaging the overdrive means that the car will automatically shift up to one gear ABOVE that set on the lever. Thus if you place the lever in position S (1),then the AT will shift up to gear 2....and when the lever is in position D (3),the AT will shift all the way to gear 4 [not labelled on the lever].
Disengaging the overdrive means the car will not exceed the set gear...infact you can drive it like a manual car starting from gear 1-to-2-to-3. However,the gear ratios of an automatic are very different from those of a manual car...gear 1[auto] is equivallent to something like gear 0.7[manual] and gear 3[auto] is equal to approx gear 3.8[manual]!!
If your car is labelled LOCK
When you engage the LOCK,you lock the car in the gear sellected on the lever...it will not shift up or down.....a very useful function when driving in heavy traffic,up steep hills,when stuck in the mud etc
When you disengage the LOCK,you release the gears and the car will shift up to one gear above that set on the lever [this is equivallent to engaging the OD]
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When driving an AT in normal conditions (no heavy traffic,no steep hills etc),you can save a lot of fuel by 'forcing' it to shift upwards earlier. You do this by releasing the accelerator momentariry and then pressing it down gently [not to the floor!!!!!!]. That way,you get it to gear 4 at about 50km/h instead of the 100km/h if you leave it to decide.
But; if you love your speed and quick accellerations,then you shouldn't be discussing fuel efficiency - whether the car is MT or AT!!!
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