A lot of the 200mpg folk fit kill switches and turn off their engines to coast. Then let it bump start itself when they need power :/ seems dangerous to me so not going that far but have been changing it into neutral for the down hills.
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I think you will find its also illegal as you are not in full control of the vehicle, perhaps someone else can confirm.
As for having a kill switch etc that is just plain stupid and these people should be removed from the road.
It depends on how you define control, you choosing to switch off the engine is fine, you're still in control so I would say not illegal per se.
What is not recommended is trying to do this sort of thing in an automatic geared car, you won't get any benefits and could wreck the drive train. On a car with power assisted steering it could be near suicidal if the kill switch stops that from working.
It is also not recommended to use coasting on a road with a lot of other road users as it will affect their driving. In that situation maintaining a steady speed is more important.
Reducing speed to match trucks will save quite a bit of fuel at a cost of a little time.
The important bit in control is to minimise distraction around you so you can concentrate on driving. Doing anything other than driving can be constituted as a distraction and thus reducing your control, so turning around to speak to kids in the back would definitely count, as would eating a biscuit or smoking a cigarette. You are also supposed to have both hands on the wheel, so keeping one hand on the gear stick and rowing through the gears could be an issue too.
Although at the extreme end of coasting it has been used to achieve MPG figures of 1,500 MPG and one team is supposedly attempting to achieve 2,000 MPG.
With automatic braking systems it is expected that lorries could drive safely very close to each other in a road train system. That would save fuel for the cars behind the first one. Similarly cars fitted with self drive systems could save by driving close to each other on motorways.
If you have to slow down then costing is better than braking later. Taking your foot off the accelerator and coasting towards junctions where you have to stop or when traffic lights change from green to amber or red will reduce consumption. Trying to time your arrival to the lights changing to green so you don't have to stop completely will also help. Steady acceleration away is better than putting your foot down hard.