Best fuel economy at 80mph - 1.6 TDi or 2.0 TDi?

[ruiatalmeida]

I Have a Octavia 1.6 TDI Combi and, driving with economy in mind, can get as low as 3.9 l/100 km(72mpg) but on normal motorway runs (Luxembourg has same top speed as France - 130km/h or 80 Mph) it does go up to up to 4.3/4.5 l/100km (66/63Mpg) depending on traffic (if there is a lot of slowing and regaining speed). What I have seen is that the old way of driving a diesel - accelerate slowly and build up speed - really does not go well with the engine, mine prefers to be quickly taken to the speed you want to be in and maintaining that afterwards. The car has now 10.000 km.

[Ultrasonic]

I Have a Octavia 1.6 TDI Combi and, driving with economy in mind, can get as low as 3.9 l/100 km(72mpg) but on normal motorway runs (Luxembourg has same top speed as France - 130km/h or 80 Mph) it does go up to up to 4.3/4.5 l/100km (66/63Mpg) depending on traffic (if there is a lot of slowing and regaining speed). What I have seen is that the old way of driving a diesel - accelerate slowly and build up speed - really does not go well with the engine, mine prefers to be quickly taken to the speed you want to be in and maintaining that afterwards. The car has now 10.000 km.

 

High load acceleration close to the torque peak is best for economy for all vehicles - Google BSFC to get an idea why if you are interested. The advice to 'accelerate slowly to improve fuel economy is a frustratingly popular lie. Where it 'works' is if it prevents drivers from accelerating quickly only to then have to brake shortly afterwards when they reach a traffic light, for example. So long as you can avoid unnecessary acceleration then as you have discovered, quick acceleration is the way to go!

Edited 14 November, 2013 by Ultrasonic

[alberg]

After putting 17" its gone up 1/2 liter on the 1.6tdi

[King Arfur]

Sorry but anyone travelling at between 50 - 56 mph in a car on the motorway ARE being a nuisance to lorries...

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Better get rid of any vehicle that travels between 50-56mph then so that they don't become a nuisance...  No, wait a minute.....  But lorries.....  So basically they are a nuisance to themselves..?!  <confused>.

 

Why does a car doing 55mph (driving very safely if we are to believe the issue of speed killing...) differ from a transit, a caravan, or a curtain-sider...?

 

I have a smaller vehicle therefore I must stay above a smaller limit?  I have more capability, therefore I must use it?

 

As soon as the lorry drivers start covering the cost of my fuel, tyres and brakes, I would be happy to travel at whatever speed they would prefer.....

[flat-eric]

I'm mad on eco driving as I work 46 miles away so around 100 miles per day.

 

I drive a 2007 Octy VRS PD170 estate with 117,000 miles on it, Shark stage 1, DPF removed, EGR removed, running standard supermarket diesel.

 

I easily get 600+ from a full tank, maxidot says 54mpg ish.

 

did a long drive once to hull from northampton with cruise set to 60mph (A1) and the guage said 59mpg!

 

 

Rob

[King Arfur]

Don't get me wrong, sometimes I will be passing others just below the 'instant ban' limit (when conditions allow) but on commutes, I am exactly the same and more reserved because of the distances driven.

[flat-eric]

Don't get me wrong, sometimes I will be passing others just below the 'instant ban' limit (when conditions allow) but on commutes, I am exactly the same and more reserved because of the distances driven.

 in that case I get 570 miles from a tank if i spank it now and then lol

[King Arfur]

 in that case I get 570 miles from a tank if i spank it now and then lol

I heard that spanking the engine every now and then is good for it...... 

 

Might even be law......

[rogerzilla]

There are some cars which give better economy at 80 than 70 despite the extra aerodynamic drag, due to quirks in the gearing and the engine efficiency at particular rpm and throttle openings (OK, diesels don't have throttles, but you know what I mean).  They are the exception though.  My old BMW was slightly better at 70 than 60 (it needed to be just over 2000rpm in 5th for best efficiency, but 60 in 4th would have meant the revs were too above the sweet spot).  I know someone with a Golf which he claims uses less fuel at 80 than at 70.

 

Also, the fuel consumption theoretically increases with the square of the speed (power requirement increases with the cube of the speed!) so 80/70 squared = 30% more consumption.  However, in the real world most drag is caused by eddies behind the vehicle and these don't necessarily behave as the simple theory suggests, so the effect on fuel consumption could be a lot more or less than 30%.

 

Our 1.4TSI goes with normal physics and uses a lot less fuel at 60 than at 70.

Edited 17 November, 2013 by rogerzilla

[Ultrasonic]

My old BMW was slightly better at 70 than 60 (it needed to be just over 2000rpm in 5th for best efficiency, but 60 in 4th would have meant the revs were too above the sweet spot).  I know someone with a Golf which he claims uses less fuel at 80 than at 70.

 

You were presumably referring to this with your throttle openings comment, but you really cannot have a meaningful discussion about mpg purely in terms of rpm, engine load needs to be considered as well. Optimal engine efficiency (minimum BSFC*) may have been at high load at just over 2000 rpm, but this does not mean that you wouldn't have got higher mpg in 5th gear at 60 mph. I strongly suspect that 60 mph in 5th would have been more efficient than 70 mph in 5th, but it is not quite absolutely impossible that 70 mph was more efficient (see more comments below).

 

*For those who are unfamiliar with this, BSFC (Brake Specific Fuel Consumption) is a measure of how much fuel it takes to generate a fixed amount of mechanical energy from an engine. BSFC varies with engine load and rpm, and 2D distributions of BSFC for an engine show how its efficiency varies over all (steady state) conditions. The following gives an example BSFC plot for an engine very similar to the 1.9TDI PD engine in my car, with a helpful explanation underneath it. Below this is a BSFC plot for a 2.0 TDI which I guess must be CR?

 

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

 

Unfortunately for the purposes of this thread I'm not aware of a reliable BSFC plot for the 1.6 TDI engine, although there is one here:

 

http://www.briskoda.net/forums/topic/249545-optimum-speed-for-maximum-mpg-octavia-16tdi-crd/#entry2927276

 

I'm not totally sure what is plotted though since the numbers on the 'contours' go up where you'd expect them to decrease.

 

Also, the fuel consumption theoretically increases with the square of the speed (power requirement increases with the cube of the speed!) so 80/70 squared = 30% more consumption.  However, in the real world most drag is caused by eddies behind the vehicle and these don't necessarily behave as the simple theory suggests, so the effect on fuel consumption could be a lot more or less than 30%.

 

I think that 30% figure relates to the change needed purely to overcome aerodynamic drag. How big the effect on mpg will be depends on what this is as a proportion of the total energy taken to drive along. It will be a lower proportion of this total for more aerodynamic vehicles with less efficient engines (e.g. sports cars).

 

As you say, you could theoretically get vehicles with very unusual BSFC distributions such that fuel economy is better at 80 mph than 70 mph in top gear. It is unlkely as for this to be the case the BSFC difference has to be large enough to compensate for the considerable difference in aerodynamic drag. I would need to see some detailed mpg data before I'd be convinced of this for any particular car.

Edited 17 November, 2013 by Ultrasonic

[Gizmo]

Better get rid of any vehicle that travels between 50-56mph then so that they don't become a nuisance...  No, wait a minute.....  But lorries.....  So basically they are a nuisance to themselves..?!  <confused>.

 

Why does a car doing 55mph (driving very safely if we are to believe the issue of speed killing...) differ from a transit, a caravan, or a curtain-sider...?

 

I have a smaller vehicle therefore I must stay above a smaller limit?  I have more capability, therefore I must use it?

 

As soon as the lorry drivers start covering the cost of my fuel, tyres and brakes, I would be happy to travel at whatever speed they would prefer.....

 

This is hardly worth replying to...but I will bite:

 

The Transit, caravan or truck will most of the time be driven as fast as possibly, be that legally or safely.

 

Why would anybody choose to put themselves in a dangerous situation (alongside a 44 Tonner) for longer than possible? they have multiple blind spots where a car can easily be ‘lost’.

 

The dawdlers are invariably the ones that when the truck starts to overtake them, start to speed up a little to either make the overtaking manoeuvre longer than is necessary or worse still speed up to the point where the vehicle that is limited has to pull back in behind them, likewise if they have had the decency to move over a lane to enable you to join the motorway / dual carriageway then don’t floor it and UNDERtake them on the inside of them... do the decent thing... adjust your speed so they can pull in front of you and then OVERtake them.

 

As for the extra cost in tyres fuel and brakes... WTF? tyre wear is not going to increase by going at 60mph on a straight motorway, likewise neither will the break wear if you are reading the road properly, and although the fuel consumption will be marginally more this surely is offset by the safety factor.

 

I was also under the impression that all learners were taught to ‘make progress’ if you are driving way below your vehicles legal speed limit and holding people up then this can hardly be classed as making progress can it? You are therefore being a hinderance and nuisance to other road users. 

[Ultrasonic]

...tyre wear is not going to increase by going at 60mph on a straight motorway,

 

I totally agree that driving so slow that HGVs need to overtake is certainly not worth it on safety and nuisance grounds. But just to be stricty accurate I think tyre wear will be greater at 60 mph than at 55 mph on a straight motorway, but I'm sure the difference is so small it really shouldn't be any sort of factor in deciding how fast to drive.

Edited 17 November, 2013 by Ultrasonic

[rogerzilla]

Regarding the point about acceleration and efficiency above, this really applies to normally-aspirated cars.  Anything with a turbo drinks fuel on boost; small turbocharged or (especially) supercharged engines return wonderful figures in the NEDC test because the test only involves unrealistically gentle acceleration (it's designed like that so that even the very slowest cars can manage to complete the cycle).  In real life, hardly enyone achieves anything like the claimed mpg and CO2 figures because the engine will be using boost every time they accelerate.  Owners of the Fiat 500 TwinAir only get about half the combined mpg.

 

Normally aspirated petrol cars are more likely to meet or exceed the EUDC figures in mixed driving and the best technique with them is, indeed, to accelerate to cruising speed as quickly as possible.

[Ultrasonic]

Regarding the point about acceleration and efficiency above, this really applies to normally-aspirated cars.  Anything with a turbo drinks fuel on boost; small turbocharged or (especially) supercharged engines return wonderful figures in the NEDC test because the test only involves unrealistically gentle acceleration (it's designed like that so that even the very slowest cars can manage to complete the cycle).  In real life, hardly enyone achieves anything like the claimed mpg and CO2 figures because the engine will be using boost every time they accelerate.  Owners of the Fiat 500 TwinAir only get about half the combined mpg.

 

If that was referring to my original comment then I'm afraid I disagree. The BSFC plot I posted a link to above was for a 1.9 TDI. The plot covers all load conditions, and high load does include turbo boost. When I accelerate at high load close to the torque peak there is significant boost pressure. My car is also old enough that I do average close to the official mpg figures (as do most people, see here). But this is definitely becoming increasingly less common as you say*, although I don't think this has anyting to do with turbos. I know absolutely nothing about superchargers though and so can't comment on that.

 

*There's a report here to this effect if anyone is interested:

http://www.theicct.org/sites/default/files/publications/ICCT_EU_fuelconsumption2_workingpaper_2012.pdf

Edited 17 November, 2013 by Ultrasonic

[King Arfur]

This is hardly worth replying to...but I will bite:

 

The Transit, caravan or truck will most of the time be driven as fast as possibly, be that legally or safely.

 

Why would anybody choose to put themselves in a dangerous situation (alongside a 44 Tonner) for longer than possible? they have multiple blind spots where a car can easily be ‘lost’.

 

The dawdlers are invariably the ones that when the truck starts to overtake them, start to speed up a little to either make the overtaking manoeuvre longer than is necessary or worse still speed up to the point where the vehicle that is limited has to pull back in behind them, likewise if they have had the decency to move over a lane to enable you to join the motorway / dual carriageway then don’t floor it and UNDERtake them on the inside of them... do the decent thing... adjust your speed so they can pull in front of you and then OVERtake them.

 

As for the extra cost in tyres fuel and brakes... WTF? tyre wear is not going to increase by going at 60mph on a straight motorway, likewise neither will the break wear if you are reading the road properly, and although the fuel consumption will be marginally more this surely is offset by the safety factor.

 

I was also under the impression that all learners were taught to ‘make progress’ if you are driving way below your vehicles legal speed limit and holding people up then this can hardly be classed as making progress can it? You are therefore being a hinderance and nuisance to other road users. 

1) tyre wear increases with an increase in speed (I have 4 vehicles I have to keep in tyres - the ones I drive the fastest wear their tyres out more quickly...).

2) brake wear is also linked to braking speed and duration (plus number of applications).

3) no idea where undertaking/overtaking on joining highways has come from.

4) I drive the M1 twice a day - I meet HGV's (and other large vehicles) travelling from between 50 and 62mph.

5) how can a driver that gets irritated miss a vehicle that has irritated them?

6) I can't compare 'safety factor' to GBP - how does one manage this?

7) I agree that reading the road properly does allow the use of engine braking over brake application, however, the number of hard braking events due to inconsiderate lane changing goes through the roof when compared to a cruise at 56mph.

8) 56mph is a legal speed - on multi-lane highways, the vehicle that wishes to progress faster simply overtakes.  Simples.

9) I drive at 04:30 for one commute, so traffic is light.

10) I drive in rush hour on the other - average traffic speed is between 30-60, so sitting on the inside lane at 56mph is hardly crime of the century....

11) I'm betting that you would be hard pushed to keep up with me on journeys that are not commutes.  Just shy of a ban is the way to go when conditions and traffic allow.

 

Unbelievably, driving habits can be adjusted for time of day, traffic weight and weather conditions, etc.  If I choose to drive conservatively on some of my journeys and whilst I am at legal speeds - it is irrelevant if other drivers are unhappy with that.  That is their problem.....  I guess those that are unhappy at me sticking to 56mph on a commute may also be peeved (tut-tutting to themselves) when I blast past at 99mph....  Can't please all of the people, etc. etc.

 

It's been fun.  Thanks.

[Gizmo]

Go and be a passenger in an articulated truck for a few hours and you will perhaps understand things a bit better (this is something IMO that EVERYONE would benefit from when they pass their test).

As i previously said, there are several blind spots in a truck where a car can easily be ‘lost’ this is where the safety factor to saving an extra 10p per journey comes in and lets be honest you can’t be that bothered about the fuel costs if you also travel at the best part of 100mph on other occasions.

 

I would be amazed if anyone notices ANY extra tyre wear by driving at 60mph compared to 56mph.... on a motorway, the biggest wear is going to come from harsh acceleration and cornering where the tyres are being pushed closer to their limits of adhesion.

 

 

 

3) no idea where undertaking/overtaking on joining highways has come from.

 

 That was purely just me venting from another things that winds me up ... inconsiderate people. 

[King Arfur]

Go and be a passenger in an articulated truck for a few hours and you will perhaps understand things a bit better (this is something IMO that EVERYONE would benefit from when they pass their test).

As i previously said, there are several blind spots in a truck where a car can easily be ‘lost’ this is where the safety factor to saving an extra 10p per journey comes in and lets be honest you can’t be that bothered about the fuel costs if you also travel at the best part of 100mph on other occasions.

 

I would be amazed if anyone notices ANY extra tyre wear by driving at 60mph compared to 56mph.... on a motorway, the biggest wear is going to come from harsh acceleration and cornering where the tyres are being pushed closer to their limits of adhesion.

 

 

 

 

 That was purely just me venting from another things that winds me up ... inconsiderate people. 

Been there - done that.  Worked for Cargo Express for a while.

 

Noticed or not, Newtonian laws cover tyres (they are not immune).

 

As for inconsiderate people, I am still trying to retro-fit 2 x Gatling guns in front grill.....   

[booke23]

As i previously said, there are several blind spots in a truck where a car can easily be ‘lost’ this is where the safety factor to saving an extra 10p per journey comes in

 

 

Although I broadly agree with not inconveniencing trucks on the Motorway, I don't really see how it is a safety issue. After all articulated lorry drivers are professionals. They are perfectly capable of overtaking. Indeed a truck limited to 56 mph will have to overtake quite a lot...........because of trucks limited to 50 mph and mobile cranes that seem to have a top speed of about 40 mph......and because of minute variations in limiters......one truck is limited to 55.5 mph and another to 56 mph...the ensuing overtake takes 3 miles causing lots of people great annoyance (although not me).   

 

My basic take on the subject is this...........We don't live in North Korea, we live in a free country. It's up to an individual to drive at whatever speed they choose within reason. 

[rw711]

Just going back to the OP regards the 1.6 CR 

We recently picked up 2011 estate with the 1.6 CR and have been astounded with how little fuel it uses. I would like to stress we live in a rural area and rarely would have a total 'urban scenario' so it may not prove the same if you have city driving conditions. - based on brim to brim I think we are getting well into high 50's mpg - its certainly not too far from the claimed combined figure- but not as good as the ideal extra urban figures quoted. 

It is however a lot better than any of our previous cars

[gregoir]

I recently drove from Birmingham airport back to Chesterfield along M42, A42 and M1, A617. Owing to volume of traffic and compulsory 50 mph on M1 and a chatty passenger, I drove a lot slower than usual ~50-60 mph. Got my best ever mpg of 62 (indicated on computer). So if you have the patience drive slower. I don't generally and usually get 50 mpg (computer and calculations agree there).

[silver1011]

I'd forgotten all about this thread!

 

The reason for the initial question was that despite my best efforts my speed always creeps back up to 70-80mph, this is mainly down to enjoying an extremely quiet road for a large chunk of my commute.

 

I went for a 2.0 in the end, in a MkII Superb and average 49-50mpg which will do for me!

[xman]

So what is the consensus? At 80mph, in real life, which uses less fuel, 1.6, greenline or 2.0?

[silver1011]

I'll never know.

 

If I ever get to try a 1.6 I'll be able to compare.

[Keithsuth]

Im running a 2020 2.0 150PS Octy Est and i drive mainly in ECO mode ive got 8K on the clock and im averaging 57-59mpg using the brim to brim calculation method.