So we've had out '07 Golf GT TDi (140bhp) for about a month now and just loving it (quite a change from our v70 but much more suitable).
I've done a 40-odd mile trip today that involved a bit of country road, A road and motorway and it averaged 48mpg.
I'm well pleased with this but I suspect there's even better to come. Are there any tips on getting the best out of these cars other than the obvious plan ahead, don't accelerate too hard, etc. i.e. what're the speed/gear combo's the engine performs best in?

I wouldn't expect much better than that to be honest without a lot of effort and driving slowly everywhere. A golf emmpty weighs more than 1500kg so theres a lot to shift.

Hmm, I've only ever really driven the bigger VAG diesels and they can be milked a little to get better than that!

As ever, try the whole idea of gaining speed on the downs and coasting as much as you can on the ups. Lay off the cruise control (unless you already have) and just back your foot off the loud pedal by 1/4" or so once you're at the speed you want to be at.

You'll easily get more than that but not until things warm up a bit and stop using the heater, light and such as much it should easily get over 50. I had 3 MK V and they were very good cars and even with less than 12k miles on them I was getting 50+ MPG with ease and I don't drive slow, drive mainly B roads. On long dual and motorway runs it was knocking on for 60MPG.

48mpg isn't great. I get that (or a bit more) out of a heavy Passat with the same engine, and I'm disappointed. It'll do a lot better on summer diesel without the anti-freezing additives in it – I get 52-54mpg in summer.

I'd say for the best results don't do lots of short trips since it'll cause the DPF to get clogged resulting in energy being wasted to clean it out. A good hard thrash at high revs periodically will help with this and many other things.

Whilst driving in town or on country roads, try and use a burst of medium throttle to get up to speed then very light throttle to almost coast up to the next corner or light. This makes a big difference. If you have a manual take a tip out of the auto's book and don't take the revs above about 2krpm unless you are in a hurry. Simlarly don't go below about 1.2krpm either, it doesn't help. Be mean with the time you are pressing the throttle not how hard you are pressing it. But don't be mean with the steering wheel. Cornering harder means keeping the speed up and therefore less acceleration and wasted energy.

You should be seeing over 50 in winter and high 50s in summer I'd say, at least. Make sure the tyres are up to pressure too.

My current Mk VI has cruise control, I get way more if I use that than I do when not using it.

I'd say use cruise control – it stops your speed creeping up when you're not watching the speedo or the fuel economy readout – which ruins mpg.

Oh it's a GT TDi – that explains a bit. Larger turbo means more back pressure I'd say – so 48 is fairly reasonable. Could still do better tho 🙂

All mine were GT TDi 140s, great cars.

I use my cruise control quite a bit too. Only problem is going up hills on the motorway when it wants to keep banging on at the same speed. I always think what it would be like if I was pedaling and tried to ride up the inclines as fast as on the flats. Its all about constant effort not speed.

Oil change between services helps keep it all healthy which has to be a good thing as does letting the car cool slightly from motorway level exertion before turned off.

Cheers guys. Don't think it's got a DPF so that's one less worry. No cruise at the moment but I'm going to get it fitted as I do like it. I've read about the winter fuel consumption thing and am already looking forward to spring!
I'll try to be a bit gentler with the accelerator but I don't want my trips to take forever, and it is a GT after all 🙂

You'll find there won't be a huge difference in fuel consumption anyway between welling it and taking it easy. My consumption has dropped massively on my current one this winter as the kids or Mrs are usually in so heaters are on a lot. Plus most of the driving is in early morning evening so lights are on too, that said the newer one has side lights in all the time anyway.

I've got a MK4 golf gt tdi 130 and I can just about get 50mpg if I drive like a granny! Usually 42mpg on the commute from Swindon to Taunton which is 90% motorway.

My vagcom cable has turned up so I'm going to test the MAF shortly and looking at cleaning the inlet manifold out as it's done 125k miles.

Like a golf but not a golf,Just because its got a set of golf clubs on the back seat doesnt make it a golf.

I'm intrigued… surely the heater doesn't use any more fuel?

The engine produces the heat anyway, surely having the heater on just means it goes into the car instead of out into the air through the radiator?

My 1.9 Tdi A3 normally knocks out 49-53 mpg whilst doing the school run twice a day, a trip to supermarket 3-4 times week and 100 mile round trip to work 3-4 times a month, so I was horrified to fill up today and calculate that my last tank worked out at 42 mpg…… Must be the winter and I must check my tyre pressures tomorrow as that's one thing I rarely check!

The engine produces the heat anyway, surely having the heater on just means it goes into the car instead of out into the air through the radiator?

It's climate control and still need power to push the air out, gone are the days of it trickling through on it's own steam.

Have another fuel strike. Empty roads and frugal driving = 65mpg in my old Golf 130TDi. Boring though.

Only problem is going up hills on the motorway when it wants to keep banging on at the same speed.

Nothing wrong with that. Probably does not use more fuel. Ok so it's working harder but not for as long. At the same revs (ie in the same gear) there's probably a similar proprtion of heat lost through the exhaust I'd guess than if you were driving on the flat, so then it should boil down to more fuel being input being cancelled out by being going up hill for less time.

As for using the heater – doesn't matter much when you've been driving for a while because the heat is being wasted.. the power required for the fan is minimal. However if it's really really cold using a lot of interior heat can mean that the engine is actually under temperature.. or if it's short trips then the engine takes longer to heat up. Although the impact of that on a diesel is probably much less than petrol.

I've got a 130 Mondeo TDI. I'm driving roughly 500 a week at the min on motorways.

Doing about 60-70mph I get around 51 mpg

Doing about 80-85mph I get 49 mpg.

The things that I have found make most difference are not having air con, heated windscreens etc on (as much as 5mpg). Under inflated tyres 3-4 mpg. Posh fuel 4-5 mpg. Cruise control doesnt make any noticable difference on my Mondeo.

Driving like a nutter I can get the average down to about 43mpg.

If your car isnt new to you, make sure your drive it quite hard for a little while ie reve it quite hard. It will loosen the engine up (if it wasn't already) and improve the mpg.

Just a few notes on my own diesel:

IT has not only glow-plugs to power (both pre and post start at times), but also water heaters, yup, thats right it has fast-heat water heaters to get the engine up to temp fast – I've not decided if this helps or hinders efficiency yet!

Banging at same speed seems to be most efficient in mine, slowing up hills and accelerating back to speed down hills seems to add a tad onto my fuel use. I'm not sure that's logical, but the difference is so small it could well fall within normal error bounds so isn't really worth worrying about.

Biggest killers are changing speeds (accelerating is taking LOTS of energy, braking just means you wasted the kinetic energy you had). Keep blowers to a min, turn demister off as soon as screen cleared, don't leave it to go off by timer.

Probably the most important – check tyre pressures.

It will loosen the engine up (if it wasn't already) and improve the mpg.

If my engine loosened up when I drove it hard I'd be worried. About all you'll do is clear some of the junk out of the intake and exhaust system, you wont make a jot of difference to the bearings/rings etc.

driving with the flow of traffic Porlock to Taunton in my Skoda 1.9td. I do lots of fuel use testing on cars at work and I am pretty good at getting factory/book figures.

The 2007 GT TDi doe snot have a DPF. The Mark VI version does.

Best way to get better mileage is too adjust your driving style to the more torquey motor.
Keep revs below 2000rpm, you don't really need more than that anyway.
Cruise control is great for the motorway but leave it off in the country lanes.
then the usual change gear early works too.

other than that, not much more you can do. driving style is the biggest contributor

Did you know you can also fill up the expansion tank inside your fuel tank (VW), to give you extra range?

Not advisable if you are planning to leave the car for some time or if the temp. varies obviously, but for a planned long trip, you can add an extra 5+ Litres to your top up.

Just give the switch just inside the neck of the fuel tank a nudge with the fuel filler nozzle and you will hear it vent.

😉

Knottie, that does not impress. If it was a regular tank average it might 🙂

CK – a technical question: In a cold petrol engine the mixture needs to be rich to encourage relibale combustion when the spark goes. Surely in a diesel this is not the case? In my old fully mechanical Passat the cold start handle advanced injection timing and raised idle revs but it did NOT add more fuel. So surely one should conclude a cold diesel does not use more fuel than a warm one, but a cold petrol does…? The mpg in my diesel is always very low for the first 5-10 minutes but I think that's due to having to drive very slowly up a steep hill then sit in traffic for a bit before hitting the open road. What do you think?

nottie, that does not impress. If it was a regular tank average it might

Or me, my current record on an 18 miles steady drive is 93mpg.

the best way to save fuel is to learn how your car uses fuel by watching an instantanious comsumption gauge and adapting your style of driving to suit. My average over the last 700ish miles is 67mpg btw.

I find those gages to be miles out showing 200mpg+, that and sitting watching that to get your best miles means you'll probably not get too far.

"I find those gages to be miles out "

yes but they do give you an indication to learn from.

a technical question: In a cold petrol engine the mixture needs to be rich to encourage relibale combustion when the spark goes. Surely in a diesel this is not the case? In my old fully mechanical Passat the cold start handle advanced injection timing and raised idle revs but it did NOT add more fuel. So surely one should conclude a cold diesel does not use more fuel than a warm one, but a cold petrol does…?

Technically, I THINK (though I don't know for sure) you're right, but in practice I don't think it works out quite that way…

I notice I don't get quite the same fuel economy through the colder months in my Golf TDi as I do in summer. Which I think is probably more due to the heater being on a lot more, and possibly extra additives in the winter fuel.

Also remember that if the engine and gearbox oil are taking longer to warm up, that's more friction. Hence you will be pressing the accelerator pedal slightly longer and harder than you would if all the oils are warm, ergo using a little bit more fuel.

Oh, and for those who doubt, cruise control in many modern cars now is actually more fuel efficient for most people than driving without it. From my experience of recent cars fitted with cruise control, it seems to be much happier to allow the car to "run on" a little bit if it creeps over the speed, and is a bit less eager to react if the speed drops slightly under the prescribed speed. Compare that to cars say 10 years old with cruise control, that in many/most cases would snap the throttle shut if the speed crept up by 0.5mph or open it wide if the speed dropped by 0.5mph, and you got a much more "kangaroo juice" effect.

My 57 plate BMW 320D I had as a company car 18 months ago would achieve quite amazing economy (considering its size and 177bhp potential) at a steady 70mph on the motorway with the cruise control on. 65-70mpg was easily possibly. Put the cruise control on at 60mph for a stretch of dual carriageway once for maybe 12 miles. Over that 12 mile stretch it achieved 72mpg! OK, a smaller lighter car would have done even better, but this was a 1500kg family saloon with 140mph top speed and 0-60 in under 8 seconds performance!

I find those gages to be miles out

Only true accurate way to measure your MPG is of course to capture your milage and the amount of fuel used every single time you refill.

The few BMW's I've had over the years (old and new) have always been within 2-3% either way on the fuel economy computer on the car. That's not mega accurate granted, but it's close enough! It could also be the difference in human error when refilling the tank, perhaps not quite brimming it as high as you did last time… 😕

as said use cruise control, turn off as much as you can and shop around for best deal on fuel as that will be the real cost saver

Mol – as far as I'm aware you're more or less right, certainly with the older D's (Before common rail) timing was about the only thing they altered.

Take 2 un-controlled controlled D engines to one side, one hot, one cold. The cold one will have larger losses due to viscosity of oil and possibly require different timing of the injection to allow a full burn with a more viscous fuel. Once you introduce idle control to it though, you have different situations – timing, fuel and temp and RPM all interact, if you want to hold the same idle you could add more fuel or change timing, until some limits when adding more fuel just drowns the engine as the timing is off, likewise you could change the timing but within limits. But often you'll raise RPM slightly so that may require a little more fuel OR more advance, OR a bit of both. I suspect modern injected Ds work with both variables to maintain idle speeds suitable for the temperature.

In essence I think that with old hardware you might just be running with poorer economy to start, with more modern engines you can run at high efficiency and frugally when up to temp, but cold engine still requires what was effectively "normal" running on the old lump. Add that to the fact that you're crawling slowly and possibly also running water heaters and window heaters and headlights off the alternator, you're bound to get low mpg to begin with. The dash gauges are notoriously unreliable, some are taken off calculations from injection pulse width and current speeds etc, some just come from rpm, throttle position and time after start etc. In a modern ECU there are usually upward of 7 30+x30+ cell tables calculating your fuel based on sensor inputs, so it just depends which info was included. Fueling and timing calculations include variables like coolant temp, air temp, revs, manifold pressure/air flowrate, knock feedback, time after start, battery voltage, fuel pressure, rate of throttle increase/decrease, and several of these scale by each other, so more fuel would be added with the throttle-rate variable if it were done cold instead of warm etc

Only true accurate way to measure your MPG is of course to capture your milage and the amount of fuel used every single time you refill.

The average MPG ones seem very accurate, the current MPG ones are or appear very random.

Actually.. the thermal efficiency of the expansion would be worse if the cylinder walls/piston were cold since some heat would be travelling into the metal/coolant. Also if it's colder in there combustion would not be as complete (even my newer DPF diesel smokes more on startup), so overall it would seem to be running less efficiently hence requiring more fuel to achieve the same driving…

Another question: in winter, a diesel should be more efficient because the intercooler would work better, right? Resulting a colder charge in the cylinder? If it weren't for winter diesel, and snow/standing water notwithstanding, you should get better mpg the colder it gets…

Only true accurate way to measure your MPG is of course to capture your milage and the amount of fuel used every single time you refill.

The pumps at petrol stations are way inaccurate, as are the cutoff valves. So when you see a discrepancy between the gague and the pump reading, it's the gague that'll be more accurate since it's computer controlled precision electronics, not a wheezy old mechanical thing.

My 57 plate BMW 320D I had as a company car 18 months ago would achieve quite amazing economy (considering its size and 177bhp potential)

That's the beauty of diesel. You only inject in as much fuel as you need to move the car, and to make a performance car you just need to put in more air and fuel when you bury the pedal. In a petrol you have to inject in enough fuel for it to ignite under spark, and then use the throttle to drag the engine back to the speed you want, which is very wasteful. That's why performance petrol cars are very uneconomical, but performance diesels can be nearly as good as normal onces. Only the size of the turbo and the weight of the larger engines drops the economy a bit in faster diesels.

It's also why a remap to higher power doesn't affect economy in a diesel. It can even improve it due to better torque allowing higher gears to be used.

Quick note to those going on about hills:

Simple physics will tell you that it makes no difference. The amount of energy required to lift the car the required height is the same no matter what speed you are travelling at.
Since most of your energy is expended fighting air resistance and mechanical friction, it's really only your average speed and gear ratios that matter.*

*that's not to say that hills don't increase your fuel use, they do of course, but how fast you go up or down them doesn't make any difference (or rather no more difference than it would on the flat).

Yup, the delta-T would be higher for a short while.

Not all TDs have intercoolers, but your temp difference would be give better cooling. D's run lean to idle (you don't match fuel ratios like in a petrol) so if you have more dense air it won't alter the amount of fuel required to idle at a set speed, but the air will expand a bit more with the explosion so may slightly reduce the required fuel in that sense. I'm not convinced it'd be more efficient overall (due to cool engine, longer warm-ups etc).

Simple physics will tell you that it makes no difference. The amount of energy required to lift the car the required height is the same no matter what speed you are travelling at.

It's not quite that simple though, as you allude to. The actual amount of energy used will depend on drag, drivetrain losses, engine maximum efficiency point, whether or not the hill gets long and slow enough for you to need to put your foot down anyway, and whether you need to lift off completely on the down side.

Petrol pump delivery measurement is pretty damned accurate BTW, they're displacement measurement rather than your car which guesses at how much fuel the injector allowed through, not accounting for blockages or wear. Figures I've found for the petrol pumps are -0 and +0.5%. Having seen fuel injectors injecting ~15% more than they're rated at, and with fuel pressure regulators having a level of inaccuracy too, I'd suspect the fuel pump was more accurate. Though whether you can top it off to the same point or not is a different matter.

my "MKIV GT TDI PD 115" got 48mpg driving at 60mph all on motorways for 140 miles except for 10 miles of the journey, in the winter.

Lights off, aircon off, heater/fan set to position 1, stereo on. I check the oil and coolant etc. regularly.

Not sure if theres something wrong with it or its just what happens with the winter fuel?!
I'm pretty sure it'd cost more to fix than i'd ever save in fuel though.

That's why performance petrol cars are very uneconomical

Fifth Gear tested the recent Porsche 911 and got 30 mpg out of it, and i think it had well over 300bhp.

Fifth Gear tested the recent Porsche 911 and got 30 mpg out of it, and i think it had well over 300bhp.

Yes, but its fairly easy to get 50+ mpg and 300 hp out of a diesel. I can easily get 28mpg from my 300hp 2litre 4×4 petrol on a run, but likewise I can get as low as 17mpg just knocking about and I'd get nowhere near the 60+mpg my 2litre TD gets. We are comparing apples and oranges though, ultimately a performance D has no throttle losses and can potter along fairly nicely with very little fuel despite having huge potential.

Try a 6.6 litre Diesel supercar that gets 70mpg