My father always said to leave the car in gear when going downhill as gravity will turn the wheels and therefore the engine meaning that no fuel is used to keep the engine ticking over on idle, which it would if it was in neutral. The MPG meter shows — : — for both in gear and in neutral going downhill, so which is right, thoughts?

My father always said to leave the car in gear when going downhill as gravity will turn the wheels and therefore the engine meaning that no fuel is used to keep the engine ticking over on idle

Correct as there is a direct connection between the wheels of the car and the engine.

which it would if it was in neautral.

Incorrect as there is no connection between the wheels and the engine so fule is required to keep the engine running, the same as if it were stationary. Whether there is a real world difference is another matter.

I thought this was clarified years ago on a top gear test in a VW. Clarkson pointed out that whilst in neutral the fuel pump is still technically pumping fuel. If you leave it in gear but don’t use any power (Labour the engine) then it actually closes off the fuel pump as such using no fuel.

I believe this is the case

^^ that’s exactly what I was going to say, recalled from the same sauce.

Whilst the above is true, actual fuel economy depends on the gradient of the hill too. A shallower gradient can be cruised down in neutral, but the engine braking will reduce how far an in-gear car will go on the same hill.
If you have to use the brakes, leave it in gear, if you can cruise at the bottom, put it in neutral near the bottom to eek it out a bit further.

Depends on how you look at it. In an instantaneous point in time being in gear. If however you look over the course of a journey then leaving the car in gear whilst descending gives rise to a retardation force from increased pumping losses and mechanical losses through drivetrain and gearbox than being in neutral (provided crank speed is higher than idle.) Therefore a better strategy for economy (but much worse for safety I must point out so don’t do it) is to coast down hills in neutral, as provided you don’t need to brake at the bottom you will cover more distance than if you leave it in gear which will more than make up for the fuel used.

Iain

As above. Though one then engine drops below a certain threshold it will be supplied with fuel to keep it idle, so if you are trundling down a hill in 5th at 1200rpm it might still be fuelling!

actually closes off the fuel pump as such using no fuel

not specifically.

If your run downhill in neutral then the engine supplies all the fuel it needs to overcome the inertia to keep the engine running (weight of pistons, flywheel etc) if however you keep the gearbox engaged then it uses less fuel because the inertia is kept moving in part due to the “pull” of the gears. IIRC. Feel free to blow my comment out the exhaust! 😀 More geek than engineer.

My Vivaro actually says in the manual that going down a steep hill you should leave it in gear and leave the gas alone, as then the engine will stop getting fuel. Other vehicles and manuals may differ.

Quite apart from that, you should leave the vehicle in gear as otherwise you’ll be leaning hard on the brakes all the way down. In fact when driving you should never be moving and in neutral under any circumstance.

Before the fuel injection, no, the engine spoinning fast in gear creates a harder vaccum inthe manifold which sucks more air/fuel into the engine through the carbs.

Early fuel injections likewise, they just mimicked carb’s and set the fuel rate in proportion to the air rate.

Newer fuel injection is cutting the fuel more and more agressively as the throttle closes. You’ll notice it if you coast down a hill in a relatively low gear so the engine revs 3,000+ and the car slows down. As the revs drop below about 1,400 you can feel the engine braking effect drop off markedly as the fuel injection starts adding fuel again, depending on how much they were trying to get the CO2 emissions down will affect how dramaitic the effect is, in a big sports car it probably barely happens, in a eco-blue-green-netic-tronic-motion car it’ll probably cut the fuel untill the engines at it’s idle.

The fuel pump has litle to do with it, if the injector is closed then it just pressurises the fuel line and the excess is returned to the tank. You might argue it’s drainig the battery, but if the engines spinning then so is the alternator so it’s just adding to the engine braking effect.

It also counter intuitively can wreck the gearbox over time as some designs the bit’s that pump the oil round the gearbox only turn when it’s in gear (presumably to reduce frictional losses when shifting between gears and reduce the load/wear on the syncromesh cones). So if you get a RWD car towed any significant distance, get it done done on a flatbed, not one that just lifts the front wheels up.

There is a hill near my house that if I turn out of the car park and knock the car into neutral, I can coast the 2 miles down the hill, across the 1 mile flat and down the 1 mile hill into town.

If I did this in gear I run out of momentum halfway across the flat section and have to use the engine to get me across it.

My Vivaro actually says in the manual that going down a steep hill you should leave it in gear and leave the gas alone, as then the engine will stop getting fuel.

Should you switch off cruise control on a steep hill to stop the engine braking?

In past times of fiddling the mpg figures in a work vehicle in order to infuriate our supervisor (who couldn’t bear it that we could get better economy than him), at the end of our shift we used to zero the computer and then roll downhill for about 8 miles back to the office, with only the last half mile on the flat. This left our kitted up 4×4 with an average mpg of around 85!

Anyway, to answer the question, we got better results rolling in neutral than in gear, going by the vehicle’s measurements, for what they’re worth.

It’s probably fractions of pence in any case.

Should you switch off cruise control on a steep hill to stop the engine braking?

IME, most cruise control will maintain a constant minimum speed, it won’t stop you going any faster.

Jambo, a few moments of throttle on the flat might be less than a constant dribble of fuel over those four miles?

Doesn’t it depend on whether you have your foot on the accelerator? Surely driving down a hill at 70 uses more fuel than rolling down at 40 or 50 ?

‘IME, most cruise control will maintain a constant minimum speed, it won’t stop you going any faster.,

So the last cruise control you drove was a 1965 military series 2 with a hand throttle for a cruise control ?

Every recent car ive driven has had cruise control that maintained the speed i set it at regardless of terrain .

You have cruise control that brakes for you?

Ah, hang about, are you talking about automatics?

Early fuel injections likewise, they just mimicked carb’s and set the fuel rate in proportion to the air rate.

I had Bosch mechanical fuel injection on a car from the early ’80s, and even that had fuel cut-off on overrun.

Good point ….. Not sure on that only ever had cruise in big auto luxobarges.

My 2008 A4 manual had cruise that would brake if vehicle went about 3mph above set speed

My manual A6 will brake for me when cruise is set.

Every recent car ive driven has had cruise control that maintained the speed i set it at regardless of terrain .

Cougar is correct. Cruise control doesn’t apply the brakes or change gear for you – just modulates the throttle.

If it’s a steep enough hill the car will accelerate past the set speed – it has no choice. I’ve experienced it on half a dozen modern cars.

What if you’re driving here…

or maybe it does, and that’s why you’re having problems? 😉

If the car has active braking the cruise control will keep you constant irrespective of terrain and whether manual or auto. My manual 5 series has it, and will break on downhills

Ultimate fuel economy simply depends upon the road load. Moving the car takes fuel, to overcome all the drag forces that try to slow that car down.

So the least fuel is used for the least total drag.

However, as mentioned above, the least drag depends on what happens after the hill.

if we assume there is the hill and just a long straight and no requirement to stop, the greatest distance travelled for the least fuel (ie max economy) is, in order best to worst:

BEST: Engine off, neutral. Economy = infinite (fuel used zero, distance traveled = max)

Median: Engine on, idling, in neutral. (Fuel used = small amount, distance traveled = max)

Worst: Engine on, in gear. (Fuel used, very small amount, but distance travelled = min)

Very worst: Engine on, in gear, full throttle, driver dragging brakes!

In the real world, of course, you don’t generally know if you’re going to have to stop, and as soon as you do, you’re knobbled because a conventional car has no way of recovering the energy stored in it’s KE. If you stop by braking, or by using “engine braking” in either case the cars KE is lost to heat, irrecoverably.

A few of the latest cars now have an automatic “sailing” mode, where they decouple the engine and coast at speed with the lowest drivetrain losses possible.

Interestingly, one “real world” advantage of EV’s (Electric vehicles) is that because they have a two way drivetrain power transfer capability, driving one “badly” doesn’t necessarily increase energy consumption as much as for an ICE driven poorly.

I HAVE to coast neutral downhill to get anywhere near decent fuel economy in my 1st gen 1.6 BMW Mini.

Without this, according to the MPG computer, I get around 33-36 mpg per tank.

If I coast in neutral while, going downhill, or drafting other vehicles, or anticipating slowing traffic before it happens, the MPG per tank goes up to 39-41 mpg.

I’m guessing on this basis that coasting in neutral saves fuel (but is probably not a clever idea on many other fronts)

The well-reasoned bit above about gearbox oil only working while in gear has me a little worried though 😯

Google DFCO. Deceleration fuel cut off.
5th gear, no throttle input may be zero fuelling. If you had a hill then a long flat, 5th gear will slow you down on the straight but you will then have to get back on the throttle to make it to the end of the road. Neutral down that same hill will consume fuel, but you might be able to coast to the end of the road without needing to use more fuel to provide power to make it to the end if you had used dfco.
There’s a huge community of hyper milers that have figured out all the tricks.

I recommend not experimenting with this on big descents. Let’s say, just to pull a random example out off the hat: the Jotunheim in Norway. No engine braking + fully loaded mondeo estate = cooked brakes.

Might equal. Hypothetically.

Thisisnotaspoon – does what you say about getting a RWD car towed apply to manuals and autos?

I’ve just had my manual RWD towed 100miles with just the front wheels off the deck… I wasn’t aware this was an issue. AA bloke certainly never mentioned it!

Isn’t it illegal to coast downhill ?

If fuel economy mattered that much to you turning the engine off and coasting would be best however watch out for that pesky steering lock .

my ‘efficient dynamics’ thingy uses the engine braking to charge the alternator….or something like that 🙂 and the cruise control definitely applies the brakes on long descents

The great saving would come from driving UP the hill in a economical manager.
And taking your car out of gear on a down hill isn’t the safest thing to do, so please don’t.

VW (and Porsche) have now introduced ‘decoupling’ on the Touareg (Cayenne) – it detects when you’re coasting and separates the engine from the gearbox to reduce engine braking and therefore increase fuel economy.

Mine certainly reports better economy while coasting in neutral than while engine braking. Whether it’s really better or not I don’t know. But I still go down hills in gear because control is better than economy, right?

Correct.

Coasting is frowned upon in Roadcraft (well it was when I learnt anyway), which policemen and beardy IAM types both learn from.

(and I’m aware that’s sort of the opposite of what I said earlier, but needs must 🙂 )

Isn’t it illegal to coast downhill ?

I very much doubt it’s explicitly illegal. I suppose it could perhaps be considered as something like Reckless Driving if they wanted to argue that you weren’t in full control of your vehicle. Seems a bit of a stretch though.

Why both with neutral and not just dip the clutch in?