Coasting Vs In Gear Mpg?

[toyboata]

Hi all,

Now before we start - this thread is asking about the economy of coasting and NOT the safety issues associated with free-wheeling - KA-PEESH!?

So, on a modern engine/transmission, is it more fuel efficient to coast down hills?

The reason I ask is because I’ve heard that unlike "old" engines, newer engines detect the load on the engine, and reduce/adjust fuelling accordingly --- so when going downhill (in gear) and gravity is acting on the car - the ECU reduces fuelling (as the engine doesn’t need to produce as much power because gravity is helping us out) - even though the revs stay the same as we are in gear.

Is this correct?

My thoughts before hearing this was the faster you can go with lower revs the better -- so coasting down a hill at 50mph at 800 to 1000 revs will give better economy than driving down the hill in gear (usually with the foot off the throttle)

Any ideas?

Matt

[vmail]

Theres no need to coast if you get over 70MPG

[JappyBeast]

As I understand it, when going downhill in gear the engine uses no fuel at all.

But, our work issued a guide to saving petrol and they said that this isnt the case. Personally I think it is more economic to leave it in gear.

[GIDDLEPIN]

Down hill no throttle + in gear = fuel shut off

Down hill no throttle + in neutral = Engine running on tick over using fuel

[reece]

Down hill no throttle + in gear = fuel shut off

Down hill no throttle + in neutral = Engine running on tick over using fuel

What he said

[JappyBeast]

Down hill no throttle + in gear = fuel shut off

Down hill no throttle + in neutral = Engine running on tick over using fuel

What he said

I also said this!

[Karma Supra]

I agree with Les.

If I reset the average consumption meter in my car when coasting down from "motorway speeds" it goes up to 99 mpg lol

Managed to average 34 mpg on the way back from Germany... not bad really considering the nature of the car ;) - and before the aygo lot say they get twice as much (as PaulT did) I point out that the engine is over three times bigger lol

[toyboata]

Thanks for the replies everyone,

So it would seem that it is better to leave it in gear, and i can understand the logic there...

But whatabout engine braking - does this not work agaisnt coasting?

[Mkuk0]

buy a Prius....sorted! car works it all out and does it for ya!

[Karma Supra]

Still gets less MPG around the top gear test track than "some other cars" :P

[Cyker]

Thanks for the replies everyone,

So it would seem that it is better to leave it in gear, and i can understand the logic there...

But whatabout engine braking - does this not work agaisnt coasting?

Yes, it will slow the car down, but this is why they say you should coast up to the line in-gear - Saves on brake wear and saves fuel!

Coasting in gear down a hill is also a lot safer than doing it in neutral for this reason, esp. in traffic etc.

Driving at, say, 30 in 5th or 4th will save fuel compared to doing it in 3rd, but when you let go of the accelerator while in gear you're using no fuel at all! (NB: But only if you're ABOVE the idle speed of the car, otherwise it will still use fuel to stop the engine going below the idle speed and stalling, and often a bit more fuel than just being in neutral at that same speed!)

buy a Prius....sorted! car works it all out and does it for ya!

I don't understand why they don't make the Prius run on Diesel; As it stands the Prius' mpg, running costs and emissions are still a lot worse than a conventional common rail turbo diesel...

Heck, I'm getting mpg in the low/mid 60's in my D4D Yaris and that is with mostly city driving! (The in-gear coasting really helps save fuel there lemme tell you!)

[duncerduncs]

I don't understand why they don't make the Prius run on Diesel; As it stands the Prius' mpg, running costs and emissions are still a lot worse than a conventional common rail turbo diesel...

Citroen have been working on a diesel hybrid for a while but I understand there are some issues with the constant stopping and starting of the engine (It's hard enough with a 20 cylinder engine!).

As for the running costs of the new Prius, what other family hatchback has a £0 VED?

[toyboata]

Yes, it will slow the car down, but this is why they say you should coast up to the line in-gear - Saves on brake wear and saves fuel!

Coasting in gear down a hill is also a lot safer than doing it in neutral for this reason, esp. in traffic etc.

Driving at, say, 30 in 5th or 4th will save fuel compared to doing it in 3rd, but when you let go of the accelerator while in gear you're using no fuel at all! (NB: But only if you're ABOVE the idle speed of the car, otherwise it will still use fuel to stop the engine going below the idle speed and stalling, and often a bit more fuel than just being in neutral at that same speed!)

Ahh right - the penny has just dropped - i get it now...

So either way - its best left in gear ;)

Thanks all

Matt

[PaulT]

So either way - its best left in gear ;)

But bear in mind if you use engine braking:

* No ABS

* No brake lights

* A fail on your driving test!

Paul.

[Aurissimo]

So either way - its best left in gear ;)

But bear in mind if you use engine braking:

* No ABS

* No brake lights

* A fail on your driving test!

Paul.

YES ABS!!! , ABS is not afected by how you select gears or clutch position...ABS Sensor works on wheel speed sensors and brake system is independant of engine function...

No brake lights is not correct either, as you could touch the brakes enough to trigger the light switch and apply barely any brake friction (light comes on very quickly before brakes are fully engaged.

And this would apply to both cases described above...

[Cyker]

I don't understand why they don't make the Prius run on Diesel; As it stands the Prius' mpg, running costs and emissions are still a lot worse than a conventional common rail turbo diesel...

Citroen have been working on a diesel hybrid for a while but I understand there are some issues with the constant stopping and starting of the engine (It's hard enough with a 20 cylinder engine!).

As for the running costs of the new Prius, what other family hatchback has a £0 VED?

Yeah, I always wondered about that too; EVERYBODY says that repeatedly starting/stopping your engine will kill it faster, yet this is exactly what start/stop and hybrid vehicles do! I assumed that they'd figured some trick, e.g. power the fluid/lubricant pumps etc. off something else to minimize damage to the engine, but that's just an unfounded assumption...!

[Karma Supra]

The Battery will fail long before the engine does, and by that time It'll cost more than the car is worth to replace anyway, so I think they just didn't care.

[vmail]

I have to disagree with Les

If fuel cut off how can the engine still do its stuff?

[toyboata]

whos les? what "stuff"?

[Cyker]

I have to disagree with Les

If fuel cut off how can the engine still do its stuff?

What stuff do you mean?

All ancillaries; AC, alternator, fluid pumps etc. are all driven by the rotation of the engine. As long as it's turning all of that stuff should continue to work.

However, when the stop/start thing cuts power to the engine, it stops spinning so all that stuff will shut down, except maybe for electrics and other stuff powered from the Battery.

[vmail]

in the Yaris I wired up the alarm to the fuel pump

If I activate the alarm while driving the car stops

[Mkuk0]

I get the feeling there are some here who don't like the Prius?

Don't knock it til ya try it I say!! :P

[Heidfirst]

I don't understand why they don't make the Prius run on Diesel; As it stands the Prius' mpg, running costs and emissions are still a lot worse than a conventional common rail turbo diesel..

I would imagine that the US market has a lot to do with - it's the main 1 for hybrids but they don't like diesel atm.

[Mkuk0]

I think it may be something to do with how the hybrid electronics starts/stops the combustion engine, wont work on a diesel. I have no idea of the details though, so don't quote me on that!