[constipated]

I've been doing a bit of an experiment with my diesel i30.

Up until recently I adopted the approach of trying to get into the highest gear possible (that the car would comfortably do) for whatever given speed. This is what had worked best with my previous petrol car to give the best fuel economy. This meant 4th gear for 60km/hr, 5th gear for 70km/hr, and 6th gear for 80km/hr. A lot of the time I'd be keeping the revs about 1600rpm, the lower limit of where the car feels comfortable and is able to pull if needed.

Using this strategy I've been getting about 5.6-5.7 L/100km on the trip computer and in reality 5.8- 5.9L/100km on calculation. Average speed around 38 km/hr.

However, I've been wondering if it would be better to keep revs closer to the lower limit of the flat peak torque curve, say 1900rpm.

For the whole tank, I've been conscious to try to keep revs around 1900. Holding a gear till 2300rpm or so, so that the higher gear falls down to around 1900rpm. So using 4th gear up to 70km/hr sometimes, and sticking to 5th gear for 80km/hr, and only using 6th if 90km/hr or above. Average speed still 37-38km/hr for this tank, but 350km into it, the trip computer is saying 5.2L/100km.

Could it possibly be better to keep revs closer to 1900rpm than 1600rpm for economy?

[Phil №❶]

constipated,

I guess the proof will be in the pudding. I think I can feel the turbo cut in at around 17-1800 rpm, so it's interesting that your experiment is in fact using the turbo boost a little. My understanding of torque is the engine's ability to pull or move an object. If you stay in that band, I guess the engine operates at maximum efficiency and could use less juice doing it. The interrelationship between torque and power is a very interesting one, perhaps Alan might like to write something about it, having an engineering background. 

[Asterix]

That's very interesting.

Could it be that you've let it labour too much..?

To me it makes sense that the engine delivers more power when the turbo is helping just a little bit.

As I have the old 5 speed version, my car is doing approx 1800 revs at 80 km/h in 5. gear, but I also shift gear at 2200-2300 revs most times.

I reset my L/100 when my car reached the 150.000 km. For the last 1600 km I have done 5.0 L/100 km 

[Dazzler]

Hi Con,

I would try running just a couple of hundred revs higher and see what it does to the economy..

[Waja]

I always try to get into 5th as fast as I can and shift around the ~2000rpm mark (yeah...I do have a little bit of a lead foot as well) but also one thing to remember is the fuel cut off when you don't apply any throttle so I tend to coast in a lower gear rather than clutch coasting or having the engine attempt to pull the revs up. Seems to work for me since I get 4.3L/100km  But I also have a tuning box and K&N apollo CAI which could alter fuel efficiency compared to those without one. Also, do remember that fuel economy also depends on the tyre pressure where when I used to use 32 all round, I got around 5.7L/100km but with 38 front and 40 rear I get 4.3L/100km.

[constipated]

I always try to get into 5th as fast as I can and shift around the ~2000rpm mark (yeah...I do have a little bit of a lead foot as well) but also one thing to remember is the fuel cut off when you don't apply any throttle so I tend to coast in a lower gear rather than clutch coasting or having the engine attempt to pull the revs up. Seems to work for me since I get 4.3L/100km  But I also have a tuning box and K&N apollo CAI which could alter fuel efficiency compared to those without one. Also, do remember that fuel economy also depends on the tyre pressure where when I used to use 32 all round, I got around 5.7L/100km but with 38 front and 40 rear I get 4.3L/100km.

You' ve left out one piece of important information. What's your average speed per tank?

My tyre pressures are generally 38 psi all round.

[Ultralights]

im averaging about 5.1, tyres at 38 PSI, but my average sped commuting is around 50Kph.   i dire to keep revs no higher than 2000 rpm, and also having a ultraguage, i try to keep boost below 4PSI.

[constipated]

Now 730km into the tank, and trip computer consumption for this tank down to 5.1 L/100km. Average speed still 37km/h. True reading would probably be about 5.3 calculated based on previous experience

This is the best economy I've gotten since before my 1000km service when I was driving it this way (ie sticking to lower gears because at that time the car was so rough with low revs). After that I tried to keep revs as low as possible and most of my economy was around 5.6-5.7 on the computer and 5.8-5.9 calculated with same average speed.

I definitely believe keeping revs up higher is the reason.

Anyone else care to give out a try over a tank, particularly if up till now you've been adopting the low revs strategy.

I've just had a fill and a reset and done a small 135 km trip today. Most of the trip was showing between 3.9 and 4.0 and finished up at 4.1 l/100km when I hit the traffic again near home. Average speed was 53 km/h.

I practice the lowest is best idea and while I've not tried recently I discovered earlier on when I got the hang of the diesel that higher revs only increased the consumption for me.  And I mean only marginally, I don't think I've ever hit 4k at all yet.

Having said that there are also some moments when I need to press a little harder to pass or just for fun on a hill.

Interesting experiment though.

[Phil №❶]

Pip,

Have you calculated manually to verify the results 

[rustynutz]

Now 730km into the tank, and trip computer consumption for this tank down to 5.1 L/100km. Average speed still 37km/h. True reading would probably be about 5.3 calculated based on previous experience

This is the best economy I've gotten since before my 1000km service when I was driving it this way (ie sticking to lower gears because at that time the car was so rough with low revs). After that I tried to keep revs as low as possible and most of my economy was around 5.6-5.7 on the computer and 5.8-5.9 calculated with same average speed.

I definitely believe keeping revs up higher is the reason.

Anyone else care to give out a try over a tank, particularly if up till now you've been adopting the low revs strategy.

I believe the key is to stay pretty much within the max torque range and don't let the revs drop that much that things start to get rough.

[constipated]

I believe the key is to stay pretty much within the max torque range and don't let the revs drop that much that things start to get rough.

That is what is interesting. Having the car at 1900rpm in 5th at 80km/hr seems to be more economical than 6th at 1600rpm at 80km/hr.

Even in 6th, the car is not labouring. But it is certainly zippier and more responsive in 5th just at the start of peak torque range.

Pip,

Have you calculated manually to verify the results 

No, only in the sense that I've driven over 1200 on a tank. Even if my trippy figures are in error I'm happy that at least they will be consistent so that if I get 3.9 l/100km now and again (never been less on any reasonable run) but other times it's up to 4.7 l/100km I can put it down to my driving/conditions/fuel etc, and be happy. Even 5.n is good enough and sometimes I get that when I've not had a chance to be economical.

I've never seen 6.n though, even on the way home from picking up new, IIRC it was around 5.9 and I watched that slowly go down over the next few weeks. I remember being excited to see 5.2, I thought I had arrived, but it just kept tumbling. I'm certain it is now going the best it ever has, and maybe will, at just on 50,000 km.

There are a lot of variables but I'd also like to think I ran it in well even though I did use the "baby it" method.  I read some very convincing arguments from some on here that the rings will bed better if put under some pressure from more a spirited break-in. I wondered if I'd perhaps failed to keep up with the latest practice. Whether or not my method was lacking the engine runs beautifully and uses zero oil between changes - at least I can't measure it on the dipstick.

[Asterix]

Hey Pip

Amazing low fuel consumption you got there. 

I do envy you that.

[Phil №❶]

Yes Pip,

Regardless of the calculations, no one can argue @ 1200 kms / tank, that's great. 

Whoever built your engine must have been a perfectionist. 

[Doggie 1]

  That's great.
I've never got that much in mine 

Getting back to the original topic... my gearshift points when not in traffic (I don't like to hold anyone up really) are:
1 -> 2 generally at 1500 (just to get a roll really - and if already moving never ever reselect 1st. Start in 2nd on even the smallest decline)
2 -> 3 at 2500 (there's a bit of a step up to 3rd)
3 -> 4 at 2000
4 -> 5 at 1600+ on highway (generally hold 4th around town)

If ever I find myself in unsympathetic traffic I bump these by only 500 rpm or so and just push the accelerator a bit more.

The real measure I use is not how much I press the accelerator nor where I swap gears but rather how much or how little I use the brake. I judge all slow-downs so that I either coast in neutral or in gear if I need to wash speed at the same time. This includes downhill on highways at speed - a practice not accepted by some who believe it unsafe. There are plenty of "advanced" techniques that are unsafe. 

My economy driving will try to waste as little momentum as possible by avoiding slowing on the over-run or worse with the brakes.

My brake pads last forever. My last car did over 200,000 km and I can't remember now whether I did or didn't change the pads... must have but probably only once.

This is the secret, not wasting the energy you have put into the car is more important (to economy) than not putting the energy in to start with.

To sum up: learn to drive without using the brakes except for stopping the last few metres at the lights. Have in mind that every time you press the brake pedal you are using your fuel not to progress your car but instead to wear out the pads.

Apologies for preaching. Some here might not have thought about such things however.

[Phil №❶]

I do the same thing too Pip, last car, I changed the brake pads @ 160K Kms and they still had some 3-4 mills on them. Driving without using the brakes unfortunately tends to inflame the impatient drivers out there, so I sometimes have to sacrifice economy for peace. We have a steady decline 1.3 kms to the off ramp on the freeway to my home. At the top I deselect cc and no juice all the way to the off ramp. I am often passed in the fast lane by other drivers and they, like me are eventually stopped by a red light, they gain nothing. I still need to apply brakes because the diesel auto just won't slow on its own.

I still need to apply brakes because the diesel auto just won't slow on its own.

Not much suck from the manual either I might say. It's a blessing sometimes as you can shut the fuel use with little holdup. But yeah, 3rd offers some resistance... but as you will know if you are into 3rd for the braking then you musta misjudged it.

[Phil №❶]

Does anyone know what happens on diesel deceleration, is there ANY fuel going to the engine eg idle quantity, or none at all. 

[Shambles]

Does anyone know what happens on diesel deceleration, is there ANY fuel going to the engine eg idle quantity, or none at all. 

I was once told that the fuel runoff is 100%, ie no fuel is delivered or consumed during decceleration.

I remember testing that theory by resetting my consumption meter while travelling along a motorway (several years ago) and taking my foot off the gas. Can't remember the exact mpg reading but it was in triple figures

I'm sure any CPU operated engine will use zero fuel on over-run. Only a dumb engine needs to maintain idle fuel.

[Asterix]

Hi Pip

Well, that explains a lot how you can obtain these nice figures.

I do not have the patience to drive like you do, but no doubt, if I did, I would get better fuel economy.

[AlanHo]

I judge all slow-downs so that I either coast in neutral or in gear if I need to wash speed at the same time. This includes downhill on highways at speed - a practice not accepted by some who believe it unsafe. There are plenty of "advanced" techniques that are unsafe. 

When you put the car in neutral and coast - the engine uses fuel in order to keep it at idling speed.

When you take your foot off the accelerator with the car in gear - fuel is no longer supplied to the engine - the motion of the car keeps it revolving.

Hence on a long fairly steep downhill slope it is best to leave the car in gear - rather than coast - and no fuel will be used.
If you put the car in neutral you may have to use the brake to limit the speed and the engine will use fuel to keep running.

On less steep declines - the engine will use fuel only when you use the accelerator to maintain speed.

It is a fine balance between when to coast, and when not to, for maximum economy. Any fuel saving would be marginal and I consider safety implications make it not worth coasting at any time.

[bryanj86]

I drive semi economically at times then other times i like to give it a few boots so im pumping smoke out the back! hehehe!! love it! Cleans the system hehe. most of the time i change at about 2500RPM. I dont drive for super-economy but i like to get reasonable economy. I normally average about 5.5L/100.

I judge all slow-downs so that I either coast in neutral or in gear if I need to wash speed at the same time. This includes downhill on highways at speed - a practice not accepted by some who believe it unsafe. There are plenty of "advanced" techniques that are unsafe. 

When you put the car in neutral and coast - the engine uses fuel in order to keep it at idling speed.
Yes
When you take your foot off the accelerator with the car in gear - fuel is no longer supplied to the engine - the motion of the car keeps it revolving.
Yes
Hence on a long fairly steep downhill slope it is best to leave the car in gear - rather than coast - and no fuel will be used.
You've lost me here, to conclude "best" is too simplistic. To save the most fuel you must use the method that uses the least fuel. When you coast in gear you are the wasting fuel you have already spent adding kinetic energy to the "car". The question you must ask is whether the fuel wasted (pushing the engine around) is greater than the fuel you would otherwise use idling it. I would answer a resounding yes!
If you put the car in neutral you may have to use the brake to limit the speed and the engine will use fuel to keep running.
But instead of using the brake you would put it in gear as you already determined is better in this situation because it slows you down and uses zero fuel doing it.
On less steep declines - the engine will use fuel only when you use the accelerator to maintain speed.
Which would be a reason to coast in neutral which will use hardly any fuel at all.
It is a fine balance between when to coast, and when not to, for maximum economy.
Any and all coasting in gear will waste fuel and impinge on the economy. This is not the answer!
Any fuel saving would be marginal...
On the contrary Alan, I would say significant.
..and I consider safety implications make it not worth coasting at any time.
An opinion shared by many it seems. 

To achieve greatest economy you should strive to waste as little as possible. By waste in this context I refer to the side of the economy ledger that refers to any fuel used for other than advancing the car - specifically this would be unnecessary or ill-judged coasting in gear (over-run) and braking. By paying attention to this aspect I get great economy and can still put the foot down occasionally to use up some of my "credit".

[Phil №❶]

The ability to drive for economy is an unfortunate balance between economy and practicality. I always drive for economy as much as possible, but that has to be tempered with the practicalities of mixing with other road users. If everyone else would get off the road, I'd have a lovely time minimising my fuel usage. For example, I like to drive away in my auto by gently depressing the go pedal and holding that position until  the car is in top gear. I estimate the acceleration to be about 1g so it's not excessive. In real life, some other user is behind me and I am forced to drive to the requirements of other users, this is the case more often than not. The other problem is no one in my household likes this method of driving so there is little harmony when we drive, but at least the car's nice to drive. 

[bryanj86]

I would say that only those who dont know how to drive well and who are foolish coast down a hill in NEUTRAL. VERY DANGEROUS! You have no control over the cars speed other than brakes which heat up and once heated too much..they FAIL. Also, you have better traction when you have your car in gear. I am horrified hearing people do this just to save a PHANTOM amount of fuel.

Ideling uses more fuel than coasting. Coasting uses a very small amount of fuel to keep the engine going as well as the cars forward motion.

My source, My father who has been both a diesel and petrol mechanic for 40+ years.

Please dont idle down hills, leave it in gear. Coasting uses less fuel than idle as idle is richer mixture of fuel than coasting.

[Phil №❶]

I would say that only those who dont know how to drive well and who are foolish coast down a hill in NEUTRAL. VERY DANGEROUS! You have no control over the cars speed other than brakes which heat up and once heated too much..they FAIL. Also, you have better traction when you have your car in gear. I am horrified hearing people do this just to save a PHANTOM amount of fuel.

Ideling uses more fuel than coasting. Coasting uses a very small amount of fuel to keep the engine going as well as the cars forward motion.

My source, My father who has been both a diesel and petrol mechanic for 40+ years.

Please dont idle down hills, leave it in gear. Coasting uses less fuel than idle as idle is richer mixture of fuel than coasting.

I'm not an advocate of coasting in neutral either for the reasons you've mentioned. I'm not convinced that in gear or neutral makes any difference at all if you're travelling down hill because in a diesel, you'll end up applying the brakes at some stage. In a diesel, I suspect idle fuel is consumed in gear or in neutral.

As far as a rich mixture at idle, fuel injection & ecu's look after all that. 

[Ace Demon]

When you put the car in neutral and coast - the engine uses fuel in order to keep it at idling speed.

When you take your foot off the accelerator with the car in gear - fuel is no longer supplied to the engine - the motion of the car keeps it revolving.

The engine needs power to turn no matter whether it is on overrun or coasting. When on overrun it takes it by reducing the car's potential and kinetic energy. When idling it takes it by using fuel. Overrun requires more power than idling (because of the rev difference).

If you want to reduce KE & PE because you have to slow down then overrun is perfect. But whatever you do, that KE & PE has to be replaced - by putting fuel into the engine.

On balance, overrun is usually likely to be better but it is not something for nothing.