When, for example, you're coming down a long hill and you select a low gear and use the engine braking to slow you, does the fact that you're doing a relatively high RPM mean you use more fuel than if you just used a high gear and used only the brakes to control the speed?

The way I figured was that if you're in a modern car with software controlled fuel injection, it would automatically lean-off the mixture to save fuel as there's very little load on the engine, but that's purely a guess.

Yup newer cars actually use little to no fuel while coasting.

With many cars, if you're engine braking and the engine is rotating over a certain RPM, no fuel is being injected. Also, I think there was a fuel economy discussion on here once, and iirc it was agreed upon that throttle position makes more of a difference in fuel economy than RPM does.

I suggest you read your vehicle's owner's manual, it should contain tips on economical driving which should mention coasting and the minimum RPM to cut off fuel.

This is actually part of the Austrian theoretical driver's licence exam, 1400rpm is provided as typical minimum RPM.

Fuel cut off while coasting: found in cars with electronic injection since 1980.

Depends on the fuel mapping of that particular car. Leaving it in gear will use less fuel than idle because the engine has to run fuel and air to idle so when your coasting the air going into the engine is at it's minimum.

Some makes run a very little amount of fuel more for engine preservation or cooling, but compared to on throttle it's so small it's barely comparable.

Ah right, so - on a slightly different subject to engine-braking down hills now - dropping to neutral (or pushing in the clutch) and rolling up to a junction actually uses more fuel than coasting up to it in gear?

yep. A lot more fuel.
In idle, a car uses between 0.6 Liters per hour to 1.4 liters per hour (depends on wether you got AC, and other electrical consuption).

This means that while coasting in neutral to a junction you are spending 1 liter / hour, independant of your speed. So you are using SOME fuel.
If your in gear, since the car's inertia is forcing the wheels to turn, all fuel is cut off, and the engine keeps turning as the wheels are forcing it to move. ZERO fuel is expended.

Thing is, if you are going through a very slight downhill section, and you keep it in gear, you will slow down (as the engine turning slows you down). If you put it in neutral, you will not slow down. So in some very specific cases (very long slight downhill sections) putting in neutral might be best, as if you put it in gear you will have to eventually accelerate as to not slow down to a crawl.

But generally, keep it in gear. Engine braking = free travelling.

Yup, as can be seen on my fuel computer, and mpg tests, engine breaking in gear is more economical.

I had a thread on here a whil ... it came up in there too.

Quote from morpha :

I suggest you read your vehicle's owner's manual, it should contain tips on economical driving which should mention coasting and the minimum RPM to cut off fuel.

This is actually part of the Austrian theoretical driver's licence exam, 1400rpm is provided as typical minimum RPM.

Neither the manual for my ZT or Wira mentioned this, nor did the manual for the Rover 75.

Quote from Crashgate3 :

Ah right, so - on a slightly different subject to engine-braking down hills now - dropping to neutral (or pushing in the clutch) and rolling up to a junction actually uses more fuel than coasting up to it in gear?

But of course in gear you slow down much much more - but it's usually a better idea to leave it in gear.

Quote from Jakg :

But of course in gear you slow down much much more - but it's usually a better idea to leave it in gear.

oh boy, i remember that conversation...

i think the consensus was that you could drive away faster if you left it in gear, but it actually brought up the topic of "does engine braking waste fuel", so didn't we cover this all already?

You can never cover the same topic too many times (exception - Schiccoco)

Of course the IAM* don't recommend engine braking at all, "gears are for go, brakes are for slow" :rolleyes:

*IAM, for you non-UK-ers, is the "Institute of Advanced Motorists". Basically the UK's largest driver improvement group. They do lots of advanced tuition and testing, and it's meant to be a similar sort of standard as Police driver training without the blue-light bits

*IAM, for you non-UK-ers, is the "Institute of Advanced Motorists". Basically the UK's largest driver improvement group. They do lots of advanced tuition and testing, and it's meant to be a similar sort of standard as Police driver training without the blue-light bits. As such they only condone driving techniques from the 1950s, and seem to have a barely adequate grasp of vehicle dynamics or operation. Ignore pretty much everything they say, and don't waste your time or money joining them or taking their 'test'.

Quote from Jakg :

Neither the manual for my ZT or Wira mentioned this, nor did the manual for the Rover 75.

The Focus manual doesn't mention it either,

Quote from tristancliffe :

*IAM, for you non-UK-ers, is the "Institute of Advanced Motorists". Basically the UK's largest driver improvement group. They do lots of advanced tuition and testing, and it's meant to be a similar sort of standard as Police driver training without the blue-light bits. As such they only condone driving techniques from the 1950s, and seem to have a barely adequate grasp of vehicle dynamics or operation. Ignore pretty much everything they say, and don't waste your time or money joining them or taking their 'test'.

The MSF (Motorcycle Safety Foundation) is similar here in the US. Their class is aimed at brand new riders on a wide range of bikes. However, once you get out in the real world, you realize there are much better ways of doing things than they teach. One particular example is their advocacy of heavy rear brake use. On a cruiser/chopper, this makes some sense due to the weight distribution and center of gravity height. On a sportbike, it does not, since the front brake is capable of lifting the rear wheel off the ground. Even on a standard-class bike, the rear brake is often more trouble that it's worth.

I remember that hypermiling thread. Made an habit of using some of the techniques described there and it indeed makes a difference!

Quote from Jakg :

Neither the manual for my ZT or Wira mentioned this, nor did the manual for the Rover 75.

But of course in gear you slow down much much more - but it's usually a better idea to leave it in gear.

Quote from Crashgate3 :

The Focus manual doesn't mention it either,

Well that's weird because my car is from the UK (although it's not a British car, but neither is the Focus) and the manual has 2 pages on economical driving. It's a '94 TT Supra, so if they bother giving advice on economical driving for a car that'll rarely be driven economically by design (I actually manage between 33 and 40mpg with a relaxed driving style and the occasional "leave the <insert German car> behind"), one would expect them to include such advice in manuals for significantly more economical cars