integra_man

Hello

Was wondering if anyone ITK can settle a discussion between me and a mate.

Over a few beers we were recently lamenting rising fuel costs and how much it costs to run a big car these days. Said mate proudly declared that one way he tries to counter this when town driving is to drive practically everywhere in neutral, wherever possible. This basically entails getting up to whatever speed the rest of the traffic is doing, maybe a little more if a safe enough distance behind, and then slipping into neutral and coasting. He argues it is particularly effective when going downhill or whenever you know you may have to stop shortly (i.e. for lights, a junction or an island), as this happens so frequently.

He argues that you consume less fuel and get more miles per fuel consumed, due to the fact that the car spends a lot more time at an idling rpm than a higher one. Having been a passenger several times with the guy, I have noticed him driving in this fashion and despite the obvious safety/traction issue he is actually rather good at it.

I disagree with his theory though because I always thought that when in gear and not depressing the accelerator (i.e. coasting in gear), the throttle valve is actually closed (which is why cars stall in gear). Hence that is better than being in neutral. I also read this in the Torygraph motoring section on Saturday and pointed this out to him that evening. However, his counter argument is that despite the valve still being open at idle rpm, he is able to coast further due to the lack of drivetrain resistance that actually results in quite effective braking the bigger the car. He does have a point - when you coast in neutral you do roll a lot further than when you coast in gear, even though you have less control.

I did point out that a by-product of his shopping trolley way of driving was that my brakes would last longer than his! He then argued that if you were adept in driving this way and did apply a clever couple of bits of engine braking here and there this was not so. As you can imagine, the debate is now starting to get less interesting than a Scottish cup game.

Can anyone settle this thrilling, enthralling debate as to which method gives the best fuel economy?

Crotch Rocket

The ECU on modern cars tends to stop the fueling when you lift off the accelerator therefore there is no fuel used when coasting in gear, but when you let the car idle you are still using fuel. Granted you can coast a bit further but you will be using fuel all the way.

apkent

Not sure I agree with his thinking, but surely it all depends on the road.

For example, I drive a road daily where you go from the NSL at the top of a hill, into a 40, then to a roadabout. If you drop a gear as you enter the 40 engine braking has slowed you enough by the bottom of the hill to prevent you needing to brake and you can just accelerate out of the roundabout.

If you'd been in neutral you'd have gained speed down the hill and would need to brake, change gear and accelerate out of the roundabout using more fuel.

I see it less as saving fuel though, and more just using common sense whilst driving.

ian_6301

I can support your mate's theory. In my skint student days, I somehow scraped together enough money to buy a clio 172 which had the jolly old MPG-ometer fitted.

Come the end of the month, it was in fact possible to achieve in well in excess of 60 mpg by gently accelerating to approx 50mph and then turning the engine off and crusing in neutral. You did however need balls of steel and big arms to get away with this, as not only was the power steering rendered useless, but the stored vacuum in the brakes was not a limitless resource and you would eventually end up with manual brakes too

cruising out of gear but with thwe engine at idle is much safer, although still not as good as cruising in gear.

As for fuel efficiency, I would think that out of gear is better, as the losses introduced by the drive train losses are indeed quite high. Add to this the fact that the engine is at ~1000 rpm out of gear and ~2500 in gear and therefore using only 40% of the fuel. The ECU may well cut fuel momentarily if you back off the throttle in gear, but it then opens it up again pretty soon. This can be proved by turning off your ignition while driving along - it will slow your car to a stop a lot quicker than if you just take your foot off the gas!

Better still, say B@llocks to Brown and fit an LPG conversion - fixed rate of tax for the next 4 yrs and mine for the landy cost only

Chris Type R

When I was trying to max Tracey's auto Jazz mileage it seemed more expensive to be in neutral than coasting in gear (7th). The engine revs at idle were higher than when coasting, which suggests that more fuel was being used. The achieved mileage tallied with this.

Si2k

Leave in gear for diesels.

For Petrols it really depends on the engine type (internal friction and pumping losses).

aldfort

Your mate has a point but it's really all about anticipation. Every time you brake you throw away energy from the fuel you burned. If you can drive in a manner that avoids heavy braking your fuel economy will be better.

Modern ECU's cut fuel when on the overrun so there is no longer a saving there and is presumably why Saab dispensed with the free wheel mechanism some years ago. (Basically this did automatically more or less what your mate does.)

Driving in town is always less fuel efficent than driving on the open roads due to all the stop start stuff.

Similarly hanging on to 8500 rpm for as long as possible is less fuel efficient than changing up earlier but I should also point out it's a lot more fun!

Bibbs

In neutral it's using a small amount of fuel.
Coasting in gear it's not using any.

Either way .. not something I'd care about. Does he turn his engine off at lights too? Does he put the petrol cap on as soon as the filler is out (don't lose those fumes)?

nomadicS2k

Seriously....

Neutral = idling = using fuel
Coasting in gear = idling = using fuel

Am I missing something?

integra_man

He never seems to buy the first round of drinks...