Fuel economy - Golf Mk6 1.6 TDi 105hp (Manual)

forgotmyname

I got my 5th and 6th backwards above now fixed. Below 60mph 6th gear gives worse consumption.

It maybe oversimplified, But you assume less revs = less fuel which is not the case.
Drive along at say 40mph with 10% throttle. Now come to a steep hill. That 10% does not give enough power to maintain 40mph.
So you push the throttle down to say 40%. Same speed, Same revs but the engine requires more fuel to bring the power and torque upto the level required.

I think the bicycle analogy works well, 1st gear gives you an easy life but less speed. 6th goes faster. Use 6th to go up a hill and you maybe a wheezing wreck by the time you get to the top. (maybe thats just me)

I have several OBD tools. Cheap bluetooth ones are under £15. You need a fairly fast phone or tablet/netbook though.
Cheap ELM327 tools are available. Cheapest ones need a laptop.

I know it sounds mad, But trust me i have checked several times and its repeatable.

I thought i was mad when the cars trip computer showed 6th gear at 50mph getting worse fuel consumption than 5th gear.

forgotmyname

Here... It was 7% difference.

https://forums.moneysavingexpert.com/discussion/comment/53334393#Comment_53334393

Post No.4

Ultrasonic

forgotmyname wrote: »

It maybe oversimplified, But you assume less revs = less fuel which is not the case....

I know it sounds mad, But trust me i have checked several times and its repeatable.

Have you done a test that confirms an mpg improvement in the lower gear, or just the throttle position test?

I think you may be slightly missing the point I (and cyclonebri1) raised. What you need to be looking at is the amount of fuel used per unit time and not the amount of fuel per injection (which is what the throttle position tells you (sort of, I think). Let me give an example with made up numbers just to give the idea:

Situation 1: throttle at 10% injecting 0.050 mL of fuel per engine revolution in 5th gear doing 2000 rpm. This uses 0.1000 L/min of fuel.

Situation 2: throttle at 13% injecting 0.065 mL of fuel per engine revolution in 6th gear doing 1500 rpm. This uses 0.0975 L/min of fuel.

Let me stress I am not saying this is a representative calculation, just that it is not possible to draw conculsions based soley on the throttle position; the engine speed also needs to be accounted for. How linear or otherwise the relationship between throttle possition and fuel injected per revolution is I do not know, and would also be critical.

Note that I am VERY far from knowedgeable on this subject and am not claiming to be. (Someone who is may well be along in a minute disgusted at the crude nature of my post...)

Ultrasonic

forgotmyname wrote: »

Drive along at say 40mph with 10% throttle. Now come to a steep hill. That 10% does not give enough power to maintain 40mph.
So you push the throttle down to say 40%. Same speed, Same revs but the engine requires more fuel to bring the power and torque upto the level required.

Just to add, I'm not quite sure what point you are trying to make here? It is going to take more power to drive up a hill in any gear. Which gear is going to be most economical will depend on where you are at on the BSFC plot for the car in each of the gears compared. Higher load and lower revs in the higher gear may well still be more economical. If you're not familiar with BSFC plots this gives the idea well:

http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_%28BSFC%29_Maps#Volkswagen_Jetta_TDI_1.9L_ALH_1999.5-2003

Ultrasonic

forgotmyname wrote: »

I thought i was mad when the cars trip computer showed 6th gear at 50mph getting worse fuel consumption than 5th gear.

Checking back I just spotted this - sorry I missed it before. This trip computer data is what matters and clearly does show that the highest gear isn't best at 50 mph, which as you say is surprising. How big was the difference out of interest?

forgotmyname

Yes the tests i did were backed up by the trip computer (which is only an average figure on my car so not that accurate) and also the fuelling information on the OBD reader.

But these figures change on a day to day basis depending on the air temperature, barometric pressure etc.

So i could repeat the tests and post these but, I could also get different readings on different days. So the figures wont be anywhere near yours.

The short term fuelling trim will also need to be taken into account and for my simple tests i dont need to be that accurate.

The only point i am making is that years ago they always said use the highest gear you can. But with the onboard computers this appears not to be true for modern cars.

A few people have got the new focus ecoboost model, They also found using more revs in a lower gear gives better fuel consumption.

Every car will be different though. So you need to work out whats best for yours.

Ultrasonic

forgotmyname wrote: »

The only point i am making is that years ago they always said use the highest gear you can. But with the onboard computers this appears not to be true for modern cars.

I absolutely get that which is why your post got my attention. I have in the past advised people that when not accelerating the best fuel economy could be achieved by driving in the highest gear that is not lugging the engine, so I am very interested to learn that this may be incorrect for some modern cars. I wonder if this is more likely to be the case with 6-speed cars? I guess it may be.

Re. doing tests with a trip computer, one way to do it is to find a flat, straight bit of road where you can drive at 50 mph (or whatever speed you want to test). Then get up to 50 mph, zero the trip computer, and drive at a steady 50 mph while it gives you the required average reading. Then go back and drive along the same bit of road in the same way in a different gear to do a comparison. My car provides instantaneous mpg but I find the average far more useful since the instantaneous is too unstable even when apparently driving at a steady speed on an apparently flat road. When I've done tests like this I've actually used part of my work commute on different days so as not to waste fuel, and I think differences are large enough to not worry about daily variations. Zeroing the trip computer just for a test section makes it much more sensitive rather than looking at the effect on a full trip average.

forgotmyname wrote:

Every car will be different though. So you need to work out whats best for yours.

My car has an old 1.9 TDi PD engine in it, and its trip computer shows me that it behaves as would traditionally be expected. E.g. at 30 mph on a flat road, best fuel economy is in 5th, which is better than 4th, which in turn is better than 3rd.

forgotmyname

Yeah the newer cars with seemingly ever higher gearing doesnt seem to help.

I guess 6th gear works well in a lab on rollers with no wind etc.

I think leaving the spare wheel out to save money and weight may get worse also. What will they start leaving out next?

Delivery_Driver

Ultrasonic wrote: »

I absolutely get that which is why your post got my attention. I have in the past advised people that when not accelerating the best fuel economy could be achieved by driving in the highest gear that is not lugging the engine, so I am very interested to learn that this may be incorrect for some modern cars. I wonder if this is more likely to be the case with 6-speed cars? I guess it may be.

Re. doing tests with a trip computer, one way to do it is to find a flat, straight bit of road where you can drive at 50 mph (or whatever speed you want to test). Then get up to 50 mph, zero the trip computer, and drive at a steady 50 mph while it gives you the required average reading. Then go back and drive along the same bit of road in the same way in a different gear to do a comparison. My car provides instantaneous mpg but I find the average far more useful since the instantaneous is too unstable even when apparently driving at a steady speed on an apparently flat road. When I've done tests like this I've actually used part of my work commute on different days so as not to waste fuel, and I think differences are large enough to not worry about daily variations. Zeroing the trip computer just for a test section makes it much more sensitive rather than looking at the effect on a full trip average.



My car has an old 1.9 TDi PD engine in it, and its trip computer shows me that it behaves as would traditionally be expected. E.g. at 30 mph on a flat road, best fuel economy is in 5th, which is better than 4th, which in turn is better than 3rd.

I've done a lot of tsting on this as I used to show people at work how to save fuel and most people tended to drive in too high a gear(especially on hills) which worsened the mpg like you said.
I have an older car which I can data log on and you can see the link to throttle position, maf voltage and injector duty cycle very clearly ie put your foot down, TPS voltage goes up massively, MAF voltage starts to rise quickly(obviously depending on efficiency of the engine)as more air is flowing and the injector duty also rises sharply which means a lot more fuel going in.
If I CBA I'll do some more logging to compare trying to accelerate in what I'd consider a higher gear to a set speed against doing it in what I'd consider the correct gear.As I know the size of my fuel injectors and can see the time period I can calculate exactly how much fuel each method uses.

londonTiger

LandyAndy wrote: »

My most relevant experience is of a Golf Mk4 1.9 90bhp that I had about 8 years ago. It consistently did 60 mpg regardless of conditions. I would expect a Mk6 1.6 to at least match that.

I also had a Mk4 150bhp GTi diesel. That never dropped below 55mpg.

the golfs have been getting bigger and bigger over enerations, not sure if that means they're getting heavier thoouh.

mk4 tdi can et 50mpg accordign to fuelly. I would have imagined that the more modern cars with better EPC will be slihtly better, but I guess any slight improvement in technology could easily be negated by driving skill.