Does Coasting Save Petrol?

Helen_from_yorkshire

My son asked me to drop him off in a local town earlier today but I had forgot that the petrol light had come on when I was out in the car the other day. As no petrol stations for miles I thought I'd risk it ... and I did get there and back without running out.

On the way back though, it was downhill some of the way so on certain parts of the journey I put the car into neutral and coasted. The road was downhill enough that I still managed to be travelling at speeds of 40/50mph and no traffic behind me. I did make sure the engine stayed on though as switching that off would have affected by brakes/power steering.

Did this save me any petrol though?

Dan29

Clarkson said a while ago that coasting uses more petrol than staying in gear but not accelerating.

A Google search reveals these comments backing him up, if your car is relatively modern:

Elementary mistake - your car uses fuel in neutral, because the engine ticks over. Modern cars use no fuel at all if you just take your foot off the accelerator and coast. You can often see this in a car with a fuel mpg computer. So just stay in top gear, and coast. (From http://www.mansized.co.uk/talk/thread.phtml/thread308/page1 )

Coasting out of gear or in neutral uses more petrol than slowing in gear and using engine as a brake. Most modern car shut of the fuel when engine braking as the car will not stall unlike when is neutral when fuel is required for tick over. (From http://www.fccuk.org/forum/showflat.php?Cat=0&Number=102478&Main=101766 )

an engine (diesel or petrol) will use a certain amount of fuel when coasting (ie in neutal or with clutch depressed) to keep the engine running. It's only when you're in gear, with foot off the accelerator, that no fuel is used (only applies in modern engines). (From here )

Helen_from_yorkshire

Thanks for that. Lucky I made it home !!

Dan29

Yes I do it too when I'm running low on petrol - I guess it's something we were told at a time when engines were less sophisticated and didn't have engine management computers etc, so you used to get further by coasting.

Now all I have to do is try to remember what I've posted above, so I stop doing it!

vansboy

& from a safety perspective, it's NOT a good idea to coast.

You'll have less control over the vehicle, no engine braking (which you'd have if you were to need to release the accelerator & apply the footbrake) & if the engine were to cut out/stall, you'd loose the powered assistance from the brakes servo & also power steering would not operate (assuming it's fitted)

So best to leave it in gear & be light footed - or take your foot off the accelerator on down hill runs, at least.

VB

david29dpo

i think it is actually illegal.

Lob_Rockster

david29dpo wrote:

i think it is actually illegal.

yes, this is correct - you're not in full control of the car.

You're better off learning to drive in a more frugal manner.

Dan29

popsaw wrote:

Please let me know when you are on the road so I can stay in

Why? Can anyone provide any examples of how this supposed "lack of control" would manifest itself in a modern car, where the need for engine braking has been pretty much eliminated by advances in technology leading to much better brakes than 10+ years ago?

wacko911

Modern Car while coasting (No engine):
NO Servo assisted braking
NO Power Steering
NO ABS
NO ESP
NO Airbags
NO seat belt pretensionser (saftey measure to prevent accidental dischage - Depends on model)

Say an Ambulance/Police/Fire service, appraching fast from behind, you need to take evasive action, engine power required.

Car will corner better under light acceleraction

Some idiot pulls out on you, you swerve and loose control as the back end of your car slides round, the correct action to take would be to steer in to the skid and apply power to regain control - planting the brakes will end up with you in the hedge or sliding down the road sidewards/backwards in to the path of an oncoming lorry.

To name but a few, I always coast (with the engine running), I dont understand how my enine sitting at 600rpm is using more fuel than 1800+rpm.

Dan29

wacko911 wrote:

Modern Car while coasting:
NO Servo assisted braking
NO Power Steering
NO ABS
NO ESP
NO Airbags
NO seat belt pretensionser (saftey measure to prevent accidental dischage - Depends on model)

Coasting as in dipping the clutch or putting the gearbox into neutral, not switching off the engine - clearly that would be foolish.

wacko911 wrote:

Some idiot pulls out on you, you swerve and loose control as the back end of your car slides round, the correct action to take would be to steer in to the skid and apply power to regain control - planting the brakes will end up with you in the hedge or sliding down the road sidewards/backwards in to the path of an oncoming lorry.

Not with ABS, surely?

wacko911 wrote:

To name but a few, I always coast (with the engine running), I dont understand how my enine sitting at 600rpm is using more fuel than 1800+rpm.

Your rev counter is at 1800+ when going downhill in 5th with your foot off the accelerator?

Chris4817

Aside from it being illegal (which it is) I was told once by a mechanic that it is dangerous for a new engine as it can allow unburnt fuel into the catalytic convertor which can cause it to fail.

I don't know whether this is true as I don't know too much about the workings of the modern motor vehicle!!