Thought I would share some thoughts and hopefully help some people get better economy.
I drive a Citreon Xsara Picasso 1.6HDi Desire.
I have noticed that I get much better MPG on my trip computer if I keep the car in gear as long as possible rather than dropping the clutch and coasting in neutral.
On a modern engine with an engine management system it will detect that fuel is no longer required and the supply is cut off completely.
The other day I was on a road trip and I came to a section of motorway that was very down hill. I left the engine in 5th gear and i took my foot off the gas. As i coasted in gear I was astonished to find my trip computer shoot up to 999.9 MPG, which I assume is the max it will feedback.
Since then I have been driving in such a way that I coast in neutral as little as possible and I have been getting much better figures.
I reckon I could break the "1000 miles from 1 tank of fuel" barrier soon.
My tank is 60 Liters (13.2 UK gallons). So 80 MPG (UK) should get me 1000ish. A tall order but I think its possible if I go on a long enough run.
I have the same effect in my 01 TDI Jetta. Coasting in gear cuts fuel off but if i put in in neutral and coast fuel is required to keep the motor running. Thanks for sharing the info! I think most cars will be going this rout if not now but at least in the near future.
Deceleration fuel cutoff (dfco) has been in use since modern EFI was put into use in the 1980s. It normally works above a threshold RPM, which you should be able to find with your scangauge or possibly a trip computer.
However it still takes energy to turn the engine while the throttle is shut -- more energy than it takes to turn it at idle. Thus, if you see a red light coming up, coast in gear since you're going to have to stop anyways. If you're trying to coast as long as possible (safely of course) then coast in neutral. That's usually only important if you are coasting on a hill that is so slight that you can't sustain speed in gear with the throttle shut, but you can with the car in neutral.
Additionally, coasting in gear with a manual transmission adds an element of safety.
I would say a target of 80 MPG is easy! I got 82 MPG in my old diesel using the car to it's full eco-credential! And i've posted previously on here about the people who got 126 MPG with a bit of clever driving, they also had a 1.6 diesel in a very similar sized Peugeot, probably the same engine too.
I experimented this as i live in Wales where there are hills everywhere, at one point you could coast in neutral for 6 miles without touching the accelerator. My argument was that in gear, you wouldn't have been able to do that as the engine would be braking and preventing you from going very fast. It's a tough one!
Coasting is dangerous - I wouldn't recommend it. There is also a small amount of fuel still going through the engine - else it would cut out - so won't make a big difference anyway. Better to drive safe then save a pence or two on fuel.
It's not really dangerous at all, so long as the engine is running the brakes/servo will be too, hence why when you're approaching a junction or the like, you have the clutch in so the car is in neutral anyway, just the same as if you put the gearstick in neutral, but you have to brake pretty hard still anyway.
When all the parameters are met in a car with DFCO (deceleration fuel cutoff ), the car will use zero fuel.
My 91 Cosworth , 98 Camaro and my 10 TDI Jetta has DFCO. In fact all cars that I have tinkered that have modern fuel injection and a manual transmission or variant of a manual transmission have DFCO strategies.
Bottom line when DFCO is activated, zero fuel is consumed.