Long time no speak. This may sound stupid after owning the car so long, but I've really enjoyed my 3 years with the Superb and thanks to regular long haul trips I admit I have turned into something of a 'motorway speeds' cruiser who rarely checks the economy. After an anal/paranoid first few months of measuring economy I ended up just trusting the Superb´s excellent return on fuel and stopped monitoring it years ago... 

 

Thing is, I have a 600 mile round trip I need to make on a budget this week. We´re travelling from Liverpool to Brighton and back again and I'm wanting to spend as little as possible on fuel to free up funds to spend on family while we´re there. Now I know conventional wisdom says the lower the speed the higher the economy, but the old adage of 55mph doesn't usually hold true. These days especially, there´s more to it than that and I do wonder what effect the turbo and long gearing have on helping the economy along (if at all)?

 

For example I do remember thinking several times that (counter-intuitive as it first seemed) cruising at around 78 to 80mph indicated for a long run seemed to return excellent economy compared to 55mph (when I tried it); and that´s where I imagined the turbo or long gearing were helping despite the higher speed. But I also know when I went from Liverpool > Lowestoft (a horribly long drive through East Anglia) I got almost 80mpg combined by sticking to 70mph on cruise control. 

 

Basically I'm unsure, and doubting myself, and a trip you really need to be as economical as possible isn't really the time to start experimenting with speeds v economy! So can anyone with this engine (or good experience of it) please throw me a bone? What would you suggest? We are leaving around 3am to beat all the rush hour traffic and still spend the whole day there (after a couple of hours kip for me, no doubt!). But so far as speeds, what do you recommend? 

 

I'd really appreciate any input on this, especially from those with direct experience with the 1.9TDi PD in the Superb Greenline. I know it might sound stupid or trivial to some, but it´s important to us this week. It's also our last hoorah in the car, as it's time for a new vehicle and I've ordered a 2.0 CRDTi 163HP Astra Elite with all the toys. I´ll miss the Superb but can´t wait for something faster and newer, especially those heated leather seats!

 

Many thanks in advance for any help guys.

Edited 16 October, 201312 yr by Derv

I would suggest driving at a speed that keeps the revs between 2000 and 2500. Good luck with the trip

Thanks.

It really is a question of the slower the better, assuming you're not below idle revs.  Drag (very roughly) increases as the square of velocity, and the power required to overcome this drag increases as the cube of velocity (see http://physics.info/drag/).  Yes, an engine has particular rev bands where it is most efficient but the differences are small compared to the massive increase in power required as we speed up.  There is no magic '56 mph' - it's just this was the speed selected to be the standard for fuel economy figures.

 

So the Superb will happily go along in 6th at 40mph.  To do 80mph requires 8 times the power output, although you'd get there in half the time - so you would have used 4 times the amount of energy, or fuel.  If you could legally drive your journey all the way at 40mph in top you'd be laughing!

 

Maxidot ought to give you a fair reading on this too - you can try it using your cruise control and reading average consumption at various speeds along the same piece of road.

That's really interesting, thank you! I just happened to notice in the past that the fuel consumption didn't seem to be much worse at motorway speeds than at 55mph indicated, so I stopped measuring and just 'drove'. I worked out that driving around 55mph would add just over an hour onto the drive compared to sticking to 70mph, and that's a bit much given the relatively small fuel consumption difference. So I'll probably just cruise at 70 where legal to do so and hope for the best. 

 

BTW it's an older pre-facelift model on a 60 plate, so it's only a 5 speed box.  

You're welcome! 70 sounds wise - plus it's safest to go with the traffic speed. Actually I've just calculated and it takes 50% more power to drive at 80 than it does at 70, quite a dramatic increase.  You get there a bit quicker so the amount of energy used, or fuel consumed, is 31% more.  Quite sobering when on the motorway!

 

p.s. Can't you shoehorn a 6th gear in there?  

Edited 16 October, 201312 yr by mfj197

You're welcome! 70 sounds wise - plus it's safest to go with the traffic speed. Actually I've just calculated and it takes 50% more power to drive at 80 than it does at 70, quite a dramatic increase.  You get there a bit quicker so the amount of energy used, or fuel consumed, is 31% more.  Quite sobering when on the motorway!

 

p.s. Can't you shoehorn a 6th gear in there?  

 

A sobering thought indeed, I'll remember that when I'm tempted to put my foot down in the outside lane! haha I guess about 70mph (just enough to touch 2k rpm so the DPF gets a clean as well) is my best bet then. A four hour drive is bad enough down the M40 and past London, but adding on an extra hour (at 55mph) to save about £4 (quick maths) just doesn't pay off in my books. At least at 70mph I've saved a fair whack over driving that extra 50% of fuel used at the usual motorway speed of most drivers...

Would agree with previous comments, the engine will be performing at its best, for cruising, at peek torque/rpm. The 1.9tdi is producing max pull around 1800-2200 rpm.  http://www.superchips.co.uk/curves/golfmk5tdi105bhp.pdf

You might also want to check tyre pressures, I run mine at + 4/5 psi above recommended, so 34/36psi.I have not noticed any problems with wear, over the life of the tyre, or driving characteristics and it should give you an extra 1/2 mpg.   

Would agree with previous comments, the engine will be performing at its best, for cruising, at peek torque/rpm. The 1.9tdi is producing max pull around 1800-2200 rpm.  http://www.superchips.co.uk/curves/golfmk5tdi105bhp.pdf

You might also want to check tyre pressures, I run mine at + 4/5 psi above recommended, so 34/36psi.I have not noticed any problems with wear, over the life of the tyre, or driving characteristics and it should give you an extra 1/2 mpg.   

 

Thanks very much. Yeah I always run the tyres at the top end (i.e. fully loaded recommended pressures) and just check them regularly, especially with weather changes (eg warmer weather pushing the pressures too high). I got 38,000 miles out of the factory fitted ContiPremiumContact 2s and even then they had 1.4mm left on them! So it doesn't do any harm, as you say. I also got a full service this week to make sure everything's tickety-boo and it's running on fresh oil and breathing through a clean filter. Fingers crossed!

In general, a diesel engine will give its lowest SFC (specific fuel consumption) near max torque. I have a 1.9PD (AWX) in my Mk 1 Superb which returns excellent figures at about 2000 rpm in top (33 mph/1000 rpm). VAG don't show SFC on any of their engine output curves.

 

As the Mk 1 Superb is an excellent aerodynamic shape (looks horrible though), there's not a huge loss at higher speeds. I find an indicated 80 mph (about 75) gives a very good figure - about 55mpg on a level motorway with no headwind.

 

The trip computer is a useless liar. Cruise is OK, but should be disengaged uphill otherwise it piles in wasteful amounts of fuel to pointlessly maintain speed. I fully agree with increased tyre pressures. On the Mk1, I run at 36 psi all round which sharpens the soggy steering (a bit) and reduces rolling resistance. Use the best quality tyres in good condition to maintain wet grip.

 

rotodiesel

Edited 16 October, 201312 yr by rotodiesel

You're welcome! 70 sounds wise - plus it's safest to go with the traffic speed. Actually I've just calculated and it takes 50% more power to drive at 80 than it does at 70, quite a dramatic increase.  You get there a bit quicker so the amount of energy used, or fuel consumed, is 31% more.  Quite sobering when on the motorway!

 

p.s. Can't you shoehorn a 6th gear in there?  

 

How do you work out that it takes 50% more power to do 10mph more?

 

And there is no way it would use 31% more fuel.

How do you work out that it takes 50% more power to do 10mph more?

 

And there is no way it would use 31% more fuel.

 

 

 

 

http://ecomodder.com/forum/tool-aero-rolling-resistance.php

 

surprising ?, but true 

How do you work out that it takes 50% more power to do 10mph more?

Because as I mentioned previously power required rises as the cube of velocity. So the required power ratio from 70 to 80 is 80³ / 70³, or 1.49 - i.e. 49% increase.

And there is no way it would use 31% more fuel.

Surprising, isn't it? Sent from my GT-I9100 using Tapatalk 2

Edited 16 October, 201312 yr by mfj197

In general, a diesel engine will give its lowest SFC (specific fuel consumption) near max torque.

SFC figures will give the engine speed at which it is operating most efficiently. However the exponential increase in power required overpowers any gains by accelerating to a faster speed just to get to the point of max SFC. Absolutely no point in driving faster to obtain a 10% increase in engine efficiency if doing so requires 50% more power.

The trip computer is a useless liar.

Agreed, but it is a consistent useless liar, so is actually quite useful for deriving comparisons in fuel economy in the one car.

Cruise is OK, but should be disengaged uphill otherwise it piles in wasteful amounts of fuel to pointlessly maintain speed.

Now here I'd disagree with you, not on the fuel economy side but from a safety and consideration to other road users point. Changing speed up hill and down dale can be very frustrating to other road users.Sent from my GT-I9100 using Tapatalk 2

Edited 16 October, 201312 yr by mfj197

Thanks again for all the interesting discussion, guys. The car's fully serviced and valeted, fluids topped up, tyres pumped to fully loaded values, and we'll be setting off around 3am in a couple of hours. According to the AA at 70mpg (easily achievable at 55-60mph) it'll only cost £23 each way, rising to £30 at 55mpg (70-80mph) so not bad. Here's hoping the traffic gods are kind haha

 

BTW mfj197, regarding hills and cruise I don't think he was suggesting rolling to a near halt for every gradient! I always knock cruise off for hilly stretches, simply nudging the accelerator when I lose more than 3-5mph in speed. MPG readout stays above 60mpg that way, rather than 20-30mpg on cruise. Big difference for not much loss of speed!

I tried driving at 40mph in 6th in my car yesterday.  Was registering 1,000rpm.  I think the only thing keeping it going was the anti-stall.

Just to update this thread before it dies: 

 

I did the trip to Brighton and back, and overall I got 76mpg indicated which worked out at an actual 70mpg brim to brim. That's definitely nothing to sniff at especially in a large car! Unfortunately my lease ends today and I'm swapping the Superb for a new vehicle. I went for a 63 plate Astra Elite 2.0 CDTi with 165ps, leather upholstery, and the usual toys. Something different, for sure. Skoda just didn't have competitive prices this time around so onwards I go. I'll be sad to see the Superb go though; especially with long-distance cruising ability and epic MPG like that!! 

 

Thanks to everyone who contributed. It's been a swell three years.

 

</fin>

Impressive going Derv!  Hope you enjoy the Astra too.

Michael

Thanks Michael. I took delivery yesterday and it's nice! Proper fast (especially compared to the Superb!), really relaxed and very refined. I was surprised!