does coasting save petrol

mkirkby

00ec25 wrote: »

Anyhow.
I'm persuaded - I'll have to learn to drive differently from now onwards.

Indeed.

In general, with a car which has an ECU that controls fuel (just about all petrol cars since 1991, and a far few before + most diesel cars after 2000)* coasting uses more fuel. Otherwise it tends to use less.


*generalisations obviously. There are some exceptions particularly with diesels as the rules with cats etc mainly applied to petrol.

00ec25

Dave_C wrote: »

At some point in the middle is the break-even point.

excellent comment - coasting may have its place

mkirkby wrote: »

coasting uses more fuel. Otherwise it tends to use less.

thanks but that was my start point - the DVLA leaflet made that assertion but did not explain why - I tend not to like Govt instructions that come without explanations as they are normally not in my favour and I want to make my own mind up about adhering to them once I have understood them

s_b

coasting will save fuel but heres the problem as explained to me in 19 nout blob
you coast down a long hill and as you reach the bottom a tractor pulls out from your left
if costing the car wont react as it would in gear because momentum wants to go where it was going
if in gear the engine power will help steer it round the tractor
a bit like abs

i often coast but am acutely aware of the risks and inherant clutch wear if i need to pop it into gear

one final thought i miss turning the engine off down a hill whilst coasting say through a village with my foot on the accelerator as i pass some pedestrians turn the ignition back on and cause a backfire:D

how we used to have fun before we joined the eu

Nearly_Old

Dave_C wrote: »

However on some long hills it is possible to coast at a much higher speed, say 40 mph. To maintain this with the engine engaged requires a fair bit of throttle - more than just tickover. So coasting uses less fuel to maintain the higher speed.

What about automatic cars in drive - where there is no engine braking?

Dave

I used to visit our Leeds office and came in from the ring road north of Leeds down the A58. Immediately after leaving the ring road there is approx 1km of constant downhill with a 40mph limit. I was driving an automatic 2.2hdi Citroen C5 and could select an instantaneous mpg display that updated every 1.5 secs. I used to accelerate reasonably quickly off the roundabout and then at 40 lift off. The car maintained 40mph as there is engine braking but not enough to force the box to change down and by half way down the hill the mpg figure would clinmb up to 999.99; i.e. the amount of fuel being used was too small to measure.

On boring long journeys when a decent hill comes along and it isn't going to inconvience anybody else I still try and find the "speed" that the car will maintain and get 999.99mpg.

worried_jim

Once on holiday in Lanzarote I managed to coast for 38km down a mountain in a Seat Ibiza- I saved fuel!

tbourner

If you're coasting down a hill you're likely to increase speed and need to brake, which is the worst thing ever for fuel economy. In gear on the same hill the engine braking slows you down, so even if you need a little bit of throttle to keep up to speed it's much better than coasting.

Tobster86

Foot off the accelerator in gear approach does also apply to diesels, especially high pressure electronically controlled injection types, but also older mechanical injection types with a centrifugal govenor.

On some diesels you can quite clearly hear the difference (no characteristic 'knock' when off the throttle). Infact, you can hear it in some petrols too.


'Engine on' coasting is an absolute waste of time. 'Engine off' coasting is probably not worth the extra brake, clutch and starter motor wear and has the added possible danger of the steering lock coming on depending on how your ignition barrel works.

s_b

Tobster86 wrote: »

It does also apply to diesels, especially high pressure electronically controlled injection types, but also older mechanical injection types with a centrifugal govenor.

On some diesels you can quite clearly hear the difference (no characteristic 'knock' when off the throttle).


'Engine on' coasting is an absolute waste of time. 'Engine off' coasting is probably not worth the extra brake, clutch and starter motor wear and has the added possible danger of the steering lock coming on depending on how your ignition barrel works.

ive run diesels for too many years to tell
i would never ever coast one
you dont save fuel and boy will it cost you if you have to dump the clutch quick

devonlad

What about a coast of two miles at about 65 mph thats only time I coast btw but from now on I think

Hintza

Why not take it to another level, the bush taxis in Africa will turn off the engine and the lights on any slope. Can be interesting!