HopeImRight

Will the Roomster give better MPG than the Octavia and will the Fabia do better than the Roomster simply because they are smaller/lighter ?

Or is it more complicated than that?

If I am very careful not to accelerate hard, never to accelerate uphill, allow my speed to fall away on up-slopes. use down slopes to build up speed for the next hill, --all the tricks----- I can get good MPG figures. God- it is boring though. Cruising across Spain and France and driving like a parsimonious parson I once got 57 mpg out of my 105 hp TDI Octavia. Brim to brim. But pretty heavy with gear.

Would I automatically get better with a Roomster and even better with a Fabia? Same engine same route etc.

Or conversely the same MPG and a bit more fun ?

Do the learned Gents on here have a view , please.

Taxi 219

I got 57.3mpg out of my Octy 105pd last night but have once got 61.1mpg too so iam quite chuffed with that compared to my Mk1 TDi 100PD.
As regarding your other queries iam sorry i cannot help.

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DanGB

The Roomster is proably less aerodynamic as well i'd imagine, so more drag effort.

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Obadiah

Official figuires suggest that the Roomster is slightly thirstier, the Fabia slightly less. In the real world I guess that there wouldn't be much difference.

ednmra

MPG can be amazing and so variable. I just got in from a run from the coast to Finchley, and back again, using A23, M23, M25, M40, A40, North Circular. A real mixture.

I left the mpg read-out switched on all the run, and it was soon in the low 40's. But just before getting back down to the M25/M3 junction coming home it was reading 49.8 mpg.
This is a 2.0 FSI, - nippy 2 litre petrol engine in a heavy-ish hatchback!!

I couldn't quite get it to 50mpg! But, at the M3 junction, congestion set in and that was that. Still pretty amazing, and nothing like my usual read out.

nickcoll

The harder you accelerate the greater the difference there will be between the Octavia and other models, as the Octavia is heavier and so will take more energy to accelerate it. If you accelerate gently then you will be using less energy and so taken across a whole journey the difference will be less significant. The relevant equation here is F=ma (Force = mass x acceleration).

Once moving it will largely come down to aerodynamic drag and speed when the vehicle is travelling along a level road. Mass is relatively insignificant here.

On hills you can gain a lot by freewheeling (coasting) downhill with the drivetrain disengaged. While going up hill make sure that you don't accelerate at all (use the cruise control) and travel at the lowest speed you can, as while you are lifting the mass of the car through an increasing height you are having to put much more energy into the vehicle and you want to minimise this as much as possible.

So the conclusion of this is that if you only ever accelerate very gently the more aerodynamic vehicle (Octavia, CD=0.30) will be better than the Fabia (CD=0.32) or the Roomster (CD=0.33). But if you accelerate harder then the lightest vehicle (the Fabia - 1170kg against Roomster - 1260kg and Octavia - 1350kg, all for saloons with the 1.9TDi engine) will be the most economical.

The one thing this doesn't take account of is frontal area, which may swing the aerodynamics back somewhat in favour of the Fabia, but since I can't find these figures anywhere it's impossible to know.

Horses for courses. If you spend a lot of time at a reasonable speed on a motorway then go for the Octavia. If you spend a lot of time around town or on back roads, then go for the Fabia. Don't buy a Roomster if you want excellent fuel consumption!

Oh, and one more thought - keep the trip computer set to the average mpg figure for the current journey - it's a great way of focusing your attention on how economically you are driving, whatever the car concerned! If the government/EU was really serious about increasing the efficiency with which we use energy they would make it compulsory for every car to have a display like this showing at all times in all vehicles.

Lummox

And of course just because they have the same engine doesnt mean they will have the same gear ratios or final drive.

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Octygone

Apart from coasting being illegal on the Octy it will use more fuel than taking your foot off the accelerator and leaving it in gear.......let me explain, the PD engine will have to put fuel in the engine while it is ticking over and you are coasting ...whereas if deaccelerating while in gear very little or no fuel is injected as the drive will keep the engine turning over...and using less fuel.

just to add to the list of fuel saving ideas ....don't fill the tank as it uses fuel to cart the extra around.....and keep the tyres pummped up and remove any excess weight (if you don't need it take it out.
If you don't need the air con ...turn it off.
same goes for the mirror heaters ...turn them off

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nickcoll

Lummox is quite right - this may alter things a bit as the engines will have slightly different efficiencies at different RPMs because of factors like combustion efficiency and friction.

This may swing things back in favour of the Fabia as it is lighter and so can support higher gearing for the same engine, or the Octavia when it has a six speed gearbox fitted that is not available (yet?) for the Fabia or Roomster.

Obviously either approach could lead to lower RPMs at the same road speed, which should lead to lower fuel consumption due to less frictional losses.

All of this does explain why the Octavia MPG figures are as good as they are in comparison with the others, although getting good results in real life will, as mentioned above, depend greatly on how aggressively you accelerate.

All of this focuses solely on MPG, and doesn't take account of other factors like the price you have to pay for the vehicle, which will be also a consideration for most people.

nickcoll

What regulation says that it is illegal to coast with the car out of gear in the UK? The Highway Code does not prohibit coasting. It does have a rule specifically for coasting (rule number 122) which would prohibit it if that was necessary, but all it does is caution that "it can reduce driver control".

I've been through the economy aspects before with someone else on a different thread, and the reality appears to be as follows:

1. If you take your foot off the accelerator while going down a hill the motion of the car certainly will keep the engine turning over without any fuel (because of the overrun cutoff) but the car will slow down quite quickly because the energy to turn the engine over has to come from somewhere. And you can't normally do this because of the speed at which other traffic is moving and because if you let the car slow down you only have to consume much more fuel to build its speed up again (because acceleration uses more energy than keeping a car going at the same speed). So unless it is a very steep hill most people will have to put their foot on the accelerator to keep the vehicle moving at a constant speed. In this situation the engine will have to turn over at the right RPM to correspond to the road speed which will probably be at least double the RPM at idle, which will cause the engine to consume about twice the fuel.

2. If you are travelling towards traffic lights or a junction then you can do this by lifting your foot off the accelerator and leaving the drivetrain engaged, but you will slow down much more quickly than if you disengage the drivetrain for the same reasons as above. So you can travel much further towards a junction with the drivetrain disengaged (engine running at idle) than by driving towards it and lifting your foot off the accelerator when you are close enough to the junction to get there.

You can try out the practical effects of the two approaches on the road as I have done. My observations show that you get much better gains by coasting with the drivetrain disengaged than by taking advantage of the fuel overrun cutoff. It's a good thing, and a definite improvement on not having it, but it doesn't appear to me to be as effective as coasting with the drivetrain disengaged.

Finally I would like to add that just like any driving technique you should not use coasting in any situation where it would be unsafe to do so. If you are not entirely happy to judge where you can safely use the technique then you definitely should not use it at all. I would not use the technique in heavy traffic, for example, but apart from this I can't think of many situations where you could use coasting that to do so would be inappropriate, or where braking or simply steering were not sufficient to control the vehicle if necessary.

eccleshill

The Highway Code is not in itself law but a code of practice to assist and guide drivers to comply with the law.

You are 100% correct in what the Highway Code says about coasting - see
117-126: Control of the vehicle : Directgov - Travel and transport

coasting is not per se an offence BUT

Road Traffic Act 1988
Road Traffic Act 1988 (c. 52)

See Paragraph 3 under the offence "Reckless Driving" , also paragraph 38 (7) re status of Highway code

Because the Highway code tells you coasting may reduce vehicle control, when you are coasting you are accepting a potentially lower level of vehicle control. This could be interpretted as reckless because you are doing it on purpose and in full knowledge that you have been warned that it could reduce control. So should you be involved in an accident while coasting then this would be taken into account when apportioning blame or deciding whether to prosecute. Not sure how they would find out however!

I think the acid test is the driving test - if you repeatedly coast in neutral or with the clutch disengaged I am pretty sure you would fail.

From the point of view of fuel saving however, I think that the difference between coasting or using a smidgin of throttle on a more gentle down gradient is insignificant.
If coasting down a steeper slope then fuel used to keep engine ticking over plus the increased use of brakes to compensate for loss of engine braking outweighs any fuel saving. However - we really are talking 3/5 of not very much at all

If you really want to gain fuel efficiency at this level then you should be driving a vehicle with regenerative braking - Toyota Prius for example.

nickcoll

I think that is pretty clear - as far as the Highway Code is concerned it says that "coasting CAN reduce driver control", not that it DOES reduce driver control.

Your point depends entirely upon a necessarily pedantic interpretation of the law and its interaction with the recommendations of the Highway Code. So to take that approach further, the fact that the wording is "can" means that as long as you are not coasting in any situation where it does reduce driver control, ie using the normal standards of driver assessment of the situation to make sure that you are driving appropriately to the situation, it is OK to do so.

For example on a long gentle hill on an empty motorway I think it would be very hard to argue that you are less in control of the vehicle if the drivetrain is not engaged. This is an extreme situation and every other situation should be judged on its particular characteristics, just as a driver has to do all the time when they are driving.

On your second point, it may be that the differences are small. This is not my experience when I have tried it, and others who practice hyper-miling swear by it too, so I don't think I am alone. But the key thing is that the crux of your argument, and that of others, is that when you take your foot off the throttle the fuel overrun cutoff ensures that the engine doesn't use any fuel. If you are now accepting that this is not always possible then surely that argument fails.

You can check this out by looking at the instantaneous fuel consumption in the trip computer - when I have done so (at, for example, 50 to 70MPH) I have observed that the fuel consumption is typically twice or more of that which you get if you disengage the drivetrain.

ednmra

I think a lot of mpg info is dodgy. The Prius was just mentioned above.
Last week a mate dragged me out with him while he test drove a Civic Hybrid. He particularly wants high mpg and is torn between hybrids and diesel. Honda claimed over 61 mpg for mixed driving. We did a steady cruise for an hour on clear dual carriageways and never got above low 40's mpg.

Back home we checked out web sites for comments - all much the same, even Honda dealers can't get the claimed figures, and judging from owners' comments, Honda couldn't care less. (See Auotsport's current report on eco-cars, also reported in today's Daily Mail.)
I told my mate to get an Octy diesel !!!

ovev

I've just picked up an 1.9TDI DSG eleganse estate. I bought it new and it now has less than 500 miles on it. On a short run from cold engine tonight of only 25 miles on typical back roads it got 60mpg. I did not drive it like an old granny, I followed the traffic and it was also quite hilly with quite a lot of ups and downs. 60mpg on a new engine is quite amazing.

HopeImRight

Can anyone add a little to my knowledge. When cruising behind a 40 tonner as I sometimes do--outskirts of Paris or Madrid for example looking for an exit and not interested in overtaking--I get the impression that truck drivers apply power in short bursts.

It seems to me they roll along with massive momentum give it a few seconds of welly --shown by black smoke-- then coast then repeat process . If I try it my speed drops off very fast compared to theirs. Less momentum I suppose. Obviously I'm on about level or down incline --uphill they power the thing along.

Is my reading of this anywhere near ?

nickcoll

That's about what you'd expect. If you're cruising on a motorway then there's no opportunity for regenerative braking, and the electric motor won't be used at all. So it ends up being a heavier-than-normal (because of all the hybrid stuff) car with a 1.4 litre petrol engine. Low 40's is actually amazingly good in this situation.

The only place that you'll really see the benefits is in driving around town, when the hybrid system will really come into its own. Lots of braking and accelerating from standstill, coupled with time standing still at traffic lights etc.

For anyone doing a lot of steady speed driving an Octavia (or similar) Diesel is definitely the best way to go!

eccleshill

You missed my other point - try taking a driving test - see how far you get if you coast repeatedly.

re interpretation of the law - I'm afraid that the "rules are for the obedience of fools and the guidance of wise men" approach would hold little water with the boys in blue. However, as I said before, the chances of being detected, even after an accident are so slim I think it unlikely to be tested.

Nuff said on the subject - agree to disagree eh?

ednmra

My friend wanted this test drive because a business acquaintance of his, who owns a Civic Hybrid and lives in central London, was moaning that he was lucky to get 30 mpg with his. The Honda official figure of 61+ mpg was supposed to reflect "mixed" driving, but on the owner website forums, nobody was getting anything like that under any conditions at all. You just can't win!

gregozedobe

I think I've read about something this somewhere, possibly some of the fuel economy threads here: TDIClub Forums - Powered by vBulletin

IIRC it is called 'pulsing', and the theory is that diesels get their best economy when they are working hard at lowish revs, so you boot it hard for a few seconds, get up to a reasonable speed, then select neutral and coast down until you are going too slow, then you boot it again etc etc.

Doesn't appeal to me, and if there is much other traffic around you would be quite disruptive to them. There are a number of "hyper-mileage" techniques that can be quite anti-social, and some are positively dangerous (like not braking for tight corners so you can maintain maximum momentum). I'll be satisfied with just very good (rather than fantastic) economy without causing problems to the rest of the traffic flow or risking my life

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Obadiah

Going back to the coasting bit. I wonder if the prohibition dates back many years, when cars had crash gearboxes and poor brakes. Coasting downhill would quickly fade the brakes and it could be difficult to get the car back into gear to use engine braking.

Jonah81

I average around 42MPG on a 2.0 TDI, but I'm having a lot of fun. I can easily do over 58MPG if needed, but it is BORING.

Fun>boring

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ednmra

Exactly! Never flooring the accelerator, gentle this and gentle that, eye on the computer. It IS boring. So you have to pay Gordon Brown for your fun!