Facts or myths?

[Johnb]

Interesting comments regarding choice of "mode".
Our Santa Fe has "Smart", "Comfort", "Eco" and "Sport". There are also "paddles" that I may rarely use and I doubt influence consumption unduly.
There is a distinct performance increase with "Sport" and other parameters beyond performance are said to be influenced (weight of steering for example)
Before buying my last Santa Fe I was firmly under the impression that for towing "sport" was the best mode and certainly if speed or throttle response are required it delivers very noticeably.
However, for most driving, including towing, I use "Smart" and the car decides which mode to use. (I can see the current mode indicated).
I've never used "sport" for long distance towing, nor for non towing for that matter. ( Except briefly if I want to quickly overtake for example). It does perk things up but the higher revs before a change and any other "changes" it makes must mean consumption will take a hit.
"Comfort" I've never used but I have tried "Economy" whilst towing. The main obvious difference between this and other modes is the higher ratios it uses - delays changing down. Whilst this might suggest a better fuel consumption I've never noticed any difference from "Smart" but there again, to make a meaningful result would require all the variables to be taken into account. I'm not a fan of mpg claims to be honest.
In my case the ratio of caravan weight to laden car weight and performance is good in all modes. My weight ratio in "real" terms is almost exactly 60% which allows for easy progress.
Bottom line though, has to be that dragging a big box on wheels around requires more fuel and, as caravanners, we are aware of that. As for actual mpg figures for me my average is close to 25 mpg but has been as low as 19 mpg and as high as 31 mpg when measured between fills.
The variables are significant and since these figures are from my own driving then the influence of my "style" is significantly reduced.
I would love to be able to claim figures of 30 mpg, 35 mpg etc that others seem to achieve - but I can't!

 

[Buckman]

Between our 2005 and 2012 Jeep Grand Cherokee 3.0 diesels, we got about 22mpg when towing. Solo about 32mpg. Both vehicles had a 5 speed box.

The 2018 Jeep GC with same engine as the 2012, gets about 20mpg when towing, but 38mpg when solo however it now has an 8 speed gearbox. It seems that due to it having more gears it uses more fuel when towing, but less when solo. I have not tried it in Sport or Eco modes when towing or solo.

 

[Jcloughie]

Mine has no fancy buttons, just put it in drive and go.

But it does have a metre on the dash. Keep the needle in the green to burn less fuel. To do this is simply common sense, gentle acceleration. Using engine braking and all the stuff we are always taught.

John

 

[RogerL]

Between our 2005 and 2012 Jeep Grand Cherokee 3.0 diesels, we got about 22mpg when towing. Solo about 32mpg. Both vehicles had a 5 speed box.

The 2018 Jeep GC with same engine as the 2012, gets about 20mpg when towing, but 38mpg when solo however it now has an 8 speed gearbox. It seems that due to it having more gears it uses more fuel when towing, but less when solo. I have not tried it in Sport or Eco modes when towing or solo.

Having more gears should be more economical solo or towing, at least in theory.

I find towing consumption varies considerably - depending on speed, wind direction and gradient - I seem to be averaging 22 mpg based on brim-to-brim fills, not the over-optimistic trip computer.

 

[Raywood]

I do wonder if you can have too many gears and that eight is just that. With six I have a gear for every occasion even when towing and more is probably just confusing and with an auto box it is simply spending too long choosing and swapping gears.

 

[JTQ]

When towing I will us the down paddle to up the engine revs, if I see a looming challenging climb, preempting waiting for the engine to realise it.

Plus, I do the same on long declines to provoke a bit more engine braking.

Disco 4, 8 speeder with 3.0 litre diesel motor.

 

[JTQ]

I do wonder if you can have too many gears and that eight is just that. With six I have a gear for every occasion even when towing and more is probably just confusing and with an auto box it is simply spending too long choosing and swapping gears.

Those of us who had been involved in diesel engine turbo matching and optimising, might well disagree.

 

[Buckman]

Having more gears should be more economical solo or towing, at least in theory.

I find towing consumption varies considerably - depending on speed, wind direction and gradient - I seem to be averaging 22 mpg based on brim-to-brim fills, not the over-optimistic trip computer.

Towing down the M5 from Worcester to Burnham on Sea it was just over 20mpg. Coming back solo back up M5 I had good mpg of about 38mpg. A few weeks later travelling down solo a few weeks later again very good mpg. Return mpg towing again about 20mpg.

On each occasion before each trip the car was topped up to the brim however I used the computer mpg which I find is quite accurate and corresponds with my manual calculations.

IMHO the vehicle has too many gears for economical towing and is probably shifting too often, but definitely not as bad as a 2013 Shogun I bought in 2018 where I was lucky to see 17mpg. It would be interesting to try it in the manual mode to see if there is any difference in mpg.

 

[RogerL]

Those of us who had been involved in diesel engine turbo matching and optimising, might well disagree.

I've not been involved like that, but disagree anyway! My own 8-speed autobox has a very low 1st to avoid the need for a low-ratio transfer box and 8th gear is high enough to cruise at autobahn speeds within the diesel's torque peak - with such a wide range to cover, it needs 8 gears to avoid any big gaps

 

[RogerL]

Towing down the M5 from Worcester to Burnham on Sea it was just over 20mpg. Coming back solo back up M5 I had good mpg of about 38mpg. A few weeks later travelling down solo a few weeks later again very good mpg. Return mpg towing again about 20mpg.

On each occasion before each trip the car was topped up to the brim however I used the computer mpg which I find is quite accurate and corresponds with my manual calculations.

IMHO the vehicle has too many gears for economical towing and is probably shifting too often, but definitely not as bad as a 2013 Shogun I bought in 2018 where I was lucky to see 17mpg. It would be interesting to try it in the manual mode to see if there is any difference in mpg.

Shifting too often is a function of the transmission electronics, sometimes done to optimise results in official tests - the 2017 VW Touareg had an "updated" transmission shift map which would hunt between 7th and 8th at motorways speeds whereas the earlier versions, like mine, would hold 8th without any issues - many owners complained and VW did provide a revised map for some owners but it was never completely resolved.

 

[Beachball]

My Tiguan got a 7 speed DSG when towing the caravan i use to put it in drive and then eco, when we went on a new stretch of a motorway it went in off road mode the car sorted it self out and continue in eco it did like normal mode as well, did try the paddles out around the steering wheel but never tried sport mode sometimes wish i had just to see what it would be like . we got over 29 mpg towing the van but solo i have had over 50 mpg

 

[ProfJohnL]

Some posts above have suggests that everyday drivers can attribute a few percent fuel economies to certain specific factors. In my opinion those claims may be well meant but they are not realistic, simply becasue there are so many factors that have a measurable effect on fuel economy, that its simply not possible for the ordinary driver to isolate the factors and attribute the results accurately.

Claims such as its a regular run and the only difference is a particular feature is patently not accurate. Don't underestimate the effect of other variables such as weather and road /traffic conditions. Singularly or combined they all affect the vehicle fuel economy, and why its very difficult for normal drivers even on a regular day to day run, to accurately assess their vehicles performance and attribute any differences to a single variable

Where I use the term "Fuel Economy", I specifically refer to 'distance travelled divide by the quantity of energy supplied by the fuel'.

Speed - You use more fuel to move faster both during the acceleration and steady state phases- This is the single most significant variable factor involved with fuel economy. At modest speeds for example 40 to 50mph, every 1mph slower you travel can make a few % improvement in fuel economy, and 10s% more at higher speeds. Whilst it may be possible to load a vehicle and ensure its mechanically identical (e.g. tyre pressures, aerodynamics and weight) and to recreate an exact speed profile across a journey, there are many other factors that can produce variance of greater than 1% in fuel economy over which the driver cannot fully predict or avoid on a regular journey.

Ambient Temperatures. These can affect not only combustion efficiency of the engine, but they also affect tyre performance, and the degree of heating or cooling you may desire inside the vehicle. Each of these factors can each easily introduce a single figure % change in the fuel economy of the vehicle. Also bear in mind how long you may idle a vehicle to defrost it, it all affects fuel economy.

Wet conditions - According to Michelin - "A tire in good shape can often displace around four (36L )gallons of water per second". For four tyres that's three full aquarolls tossed 2m into the air! (Edit )"That is some serious horse power and is demonstrated by the added drag you feel if you drive through a puddle". Under normal UK wet weather without standing water I'd guess the effects of rain could easily cause up to a 5% loss of MPG. Reducing speed in wet weather will also reduce the wet weather losses and add some additional safety to your journey.

Air drag/ wind resistance - because of the way that drag changes with the square of the speed even small changes in wind direction can make a significant change to the fuel economy. Just as a personal example:- In the 1970's I drove a fully loaded (30cwt) luton bodied transit van from the Midlands to Felixtow I think I averaged 25mpg. Returning empty I only averaged 16mpg. The main difference was the fact a full westerly gale was blowing all day.

12V Electrical loads - All 12V power is derived from the vehicles fuel source, use more electrical power causes reduced fuel economy - Apart from starting ICE engines, there are electrical loads that will have some measurable effect on fuel economy. Especially those items that are used for long periods such as heater fans, and lights. With some more modern vehicle there may also be other items like driving assistance, gearbox control, and steering systems. Some people have excessively loud and powerful in car entertainment systems - All of which need power supplied from the cars fuel source.

Gears - I.C. Engines have a severely limited rev range, with a non linear power and torque delivery characteristics They produce optimum economy when the engine runs at a particular narrow band of RPM. A gearbox is used to try and match the vehicles speed to engines optimum economy rev band. The narrower the rev band the more gear ratios are needed to allow optimum economy provided but it is dependant on the correct gear being selected. Modern auto boxes will be superior to virtually any human driver except for possibly the ability of a driver to predict a change that may be necessary based on forward awareness of changing road conditions.

In practice, for differences up to about 5% of , I seriously doubt any driver's ability under normal driving conditions to accurately attribute the fuel economy differences to a single contributory factor.

 

[Raywood]

I do agree that the variables do make getting fuel usage accurate. My driving routes do vary but overall I do have a measurable loss in mpg since E10 was introduced and the mileage involved is now so high it has to be the fuel as the variables will have evened themselves out. I drive no faster and do try most than ever to keep fuel consumption down, so possibly the current loss of MPG is potentially worse in real life.
Traffic levels do vary and I agree weather is a serious potential variable. To illustrate this we did a return trip from Strathclyde Park to Chester with the caravan on the back both ways with a strong Northerly wind. meaning we were driving into it going down and had it behind us coming back. The car registered 26 mpg driving into the headwind and 36 with it behind us. Even allowing for any variables with the traffic the effect is clear. On less windy journeys the difference will be there but far more subtle.

 

[Buckman]

Today for the first time since purchasing the Jeep I tried the Eco mode. Made no difference on the rev counter on a reasonable flat drive however I did notice that when pulling away it was not as sharp and lot more smoother. Same with gear changes. I have never tried the Sport mode as that is known to increase fuel consumption.

 

[RogerL]

Today for the first time since purchasing the Jeep I tried the Eco mode. Made no difference on the rev counter on a reasonable flat drive however I did notice that when pulling away it was not as sharp and lot more smoother. Same with gear changes. I have never tried the Sport mode as that is known to increase fuel consumption.

You might find Sport useful in hilly terrain - it'll change down more readily as soon as it detects any slowing down going up a hill as well as holding lower gears for longer going downhill.

 

[Buckman]

You might find Sport useful in hilly terrain - it'll change down more readily as soon as it detects any slowing down going up a hill as well as holding lower gears for longer going downhill.

Only hill near us is Malvern Hills however thanks for the tip and will try and remember the sport mode next time we go to Wales. LOL!

 

[Beachball]

You might find Sport useful in hilly terrain - it'll change down more readily as soon as it detects any slowing down going up a hill as well as holding lower gears for longer going downhill.

Might try sport mode in the Tiguan when we go to the Lake District next month and see what it like

 

[ProfJohnL]

I do agree that the variables do make getting fuel usage accurate. My driving routes do vary but overall I do have a measurable loss in mpg since E10 was introduced and the mileage involved is now so high it has to be the fuel as the variables will have evened themselves out. I drive no faster and do try most than ever to keep fuel consumption down, so possibly the current loss of MPG is potentially worse in real life.
Traffic levels do vary and I agree weather is a serious potential variable. To illustrate this we did a return trip from Strathclyde Park to Chester with the caravan on the back both ways with a strong Northerly wind. meaning we were driving into it going down and had it behind us coming back. The car registered 26 mpg driving into the headwind and 36 with it behind us. Even allowing for any variables with the traffic the effect is clear. On less windy journeys the difference will be there but far more subtle.

I'm not suggesting there isn't an economy difference between E5 and E10 fuels, the chemistry tells us there is less potential energy in the same volume of E10 compared to E5, which should exhibit a reduction in MPG. but the figures you have given don't agree with the science, which suggests there are also other factors ( such as those I have given) in play that have contributed to your observed reduction in fuel economy. Look at it another way and that is you are using more to do the same thing, and perhaps that includes using more right foot for longer to unconsciously try to restore lost acceleration.

 

[RogerL]

I'm not suggesting there isn't an economy difference between E5 and E10 fuels, the chemistry tells us there is less potential energy in the same volume of E10 compared to E5, which should exhibit a reduction in MPG. but the figures you have given don't agree with the science, which suggests there are also other factors ( such as those I have given) in play that have contributed to your observed reduction in fuel economy. Look at it another way and that is you are using more to do the same thing, and perhaps that includes using more right foot for longer to unconsciously try to restore lost acceleration.

It's a bit like the discussions for "chipping" a turbo-diesel - you will gain more power/torque and you might get better economy but only if you don't use any of the extra power/torque

 

[Raywood]

I'm not suggesting there isn't an economy difference between E5 and E10 fuels, the chemistry tells us there is less potential energy in the same volume of E10 compared to E5, which should exhibit a reduction in MPG. but the figures you have given don't agree with the science, which suggests there are also other factors ( such as those I have given) in play that have contributed to your observed reduction in fuel economy. Look at it another way and that is you are using more to do the same thing, and perhaps that includes using more right foot for longer to unconsciously try to restore lost acceleration.

My right foot has not been the problem. I have very consciously been trying to get the economy up and it has just not happened. I know the alleged scientific figures but I do wonder if they have been massaged in some way to favour the implementation.

 

[ProfJohnL]

My right foot has not been the problem. I have very consciously been trying to get the economy up and it has just not happened. I know the alleged scientific figures but I do wonder if they have been massaged in some way to favour the implementation.

I'm basing my comments on scientific information not politically motivated perspectives which you can verify if you wish, a point I made in one of my earlier replies.

 

[Buckman]

It appears that if you have a diesel engine it is always advisable to keep the tank topped up due to water contamination.

Partially filling a diesel tank with ethanol blended diesel will absorb water from the air space above the fuel faster than a full tank would. Regular diesel without ethanol will still absorb water, but with ethanol blended diesel fuel the water problem is made worse.

See https://www.tankservices.co.uk/water-in-diesel-fuel-contamination-in-gas-oil/

 

[RogerL]

It appears that if you have a diesel engine it is always advisable to keep the tank topped up due to water contamination.

Partially filling a diesel tank with ethanol blended diesel will absorb water from the air space above the fuel faster than a full tank would. Regular diesel without ethanol will still absorb water, but with ethanol blended diesel fuel the water problem is made worse.

See https://www.tankservices.co.uk/water-in-diesel-fuel-contamination-in-gas-oil/

Regular diesel is blended with up to 7% bio-diesel, hence the B7 specification - premium diesel may have a higher Cetane Number, but diesel engines don't adjust to suit the fuel, unlike petrol engines. There is no specific grade of diesel that has zero bio-diesel content.

Diesel engines have a water trap built into their fuel filter so it's important to change the filter at the recommended interval, including emptying the filter and refilling with fresh diesel - some older diesel vehicles had a drain point on the filter to remove water at more frequent intervals.

 

[Buckman]

Regular diesel is blended with up to 7% bio-diesel, hence the B7 specification - premium diesel may have a higher Cetane Number, but diesel engines don't adjust to suit the fuel, unlike petrol engines. There is no specific grade of diesel that has zero bio-diesel content.

Diesel engines have a water trap built into their fuel filter so it's important to change the filter at the recommended interval, including emptying the filter and refilling with fresh diesel - some older diesel vehicles had a drain point on the filter to remove water at more frequent intervals.

Thanks. I do and try to keep the tank on our diesel close to full most times. Not sure if our old 1996 Corolla petrol would have the same issue as generally we let it drop to 1/4 tank before refilling.

 

[otherclive]

Regular diesel is blended with up to 7% bio-diesel, hence the B7 specification - premium diesel may have a higher Cetane Number, but diesel engines don't adjust to suit the fuel, unlike petrol engines. There is no specific grade of diesel that has zero bio-diesel content.

Diesel engines have a water trap built into their fuel filter so it's important to change the filter at the recommended interval, including emptying the filter and refilling with fresh diesel - some older diesel vehicles had a drain point on the filter to remove water at more frequent intervals.

Haven’t we been around this one just a week ago. I recall an OP posted something from another forum then there was a post re plastic tanks, diesel fuel filter draining etc.