The engine is working at far fewer revs when ticking over, i.e. when coasting, compared to engaged in say 5th gear. That alone may mean it has a longer lifespan (edit: meant to say, though this doesn't necessarily hold true due to potentially greater strain on the engine or parts from, effectively, changing gear more often).
So my reasoning is that, since the engine is working at higher revs when in gear, I am either using more fuel to turn the engine at these higher speeds or I am using my kinetic energy to turn the engine at these higher speeds. So to put it simply.. I am either moving the car forward at 50mph and maintaining the revs at 1,000rpm or I am moving the car forward at 50mph and maintaining the revs at 2,000 rpm, so the first outcome is surely more fuel-efficient.
Yes, I know rpm is not a necessarily accurate way of assessing fuel consumption, but it is sufficiently accurate for this analogy.
And I'd like to point out the difference between coasting with the car in neutral, and coasting with the car in 5th gear but the clutch (fully) down - with the car in neutral, you have to faff around with the gear stick to put it in gear; with the clutch down but in 5th gear, it is literally fractions of a second to bring the clutch up and put the car back in gear, and the transition is smooth unless travelling much faster (>65). No risk of putting it in the wrong gear at all.
Yes, it
feels as though you have less control over the car - that is because the car is no longer automatically slowing you down the moment you release the accelerator, so it is as though a restriction on movement has been lifted. That is not to say you actually have less control over the car in a real-world situation.
Edit: can't say I've tried the bottom two, PMKeates. Old car so there may be random metal filings stuff at the bottom of my tank, so I try not to let it get too low, and never really considered or had anywhere to test out reverse speed.. you've got me wondering now though
