[Closed] Maximising MPG in a diesel?

but I'm a bit worried about trundling around at such low revs at other times

If you getting 40mpg from a 2.2 omega then you aren't doing bad.

I've always found I change gear less in a diesel due to the available torque low down. I usually use mine from about 1500 to 3000 revs but will go higher if accelerating hard and keep it above 2000 to keep the turbo spinning. I don't go ott on being economical though.

My theory is that lots of technology in diesel engines such as swirl flaps and EGR valves are designed to work when the engine doesn't have much load. if you floor it, all this stuff swtiches off and as much fuel is dumped into the engine as possible. so to drive it efficiently means being very gently with the acclerator, even if that means using slightly higher revs.

seems to work for me anyway

diesel omegas are known for poor fuel economy - surprised you're managing mid 40s out of it - although that may only have been the 6cyl models

Using an engine at peak torque isn't necessarily the most economical way. a good general rule of thumb is to keep revs low, without the engine 'struggling'. I'd be surprised if the peak torque is that low in the rev range though - my similarly-engined mondeo's turbo (and thus torque) kicks in around 1900rpm

It's the chain cam 4 cyclinder GM engine - sounds like a tractor but goes quite well 🙂

As I said, I've never owned a diesel before, so was unsure if continuous low rev use would be detrimental.
Seems not, so I'll continue driving like a nun.

5lab, I was surprised as well, but that's what the manual tells me.
Doesn't mention a minimum recommended rev limit for normal use though.

The mid 40's mpg figure was mostly long A road and motorway jaunts btw - am now using it for work and expect this to drop as it will see more urban use.

Cheers for all the advice.

Quite like the gentle nature of the power, but sick of changing gear all the time!

Well then don't. Dunno what you are talking about but I always change gear much less in a diesel on a windy country road than a petrol. When slowing into a corner you can stay in top and come out from 30mph if you want quite nicely - instead of needing 3rd.

You want to keep the revs as low as possible since the more time the fuel/air spends in the cylinder the more completely it'll burn, I think. Plus at higher revs you'll use less throttle - so youd burn the same amount of fuel in theory but in practice you'd be compressing more air you don't need, which is wasteful.

However if you drive at below say 1200 rpm you might not be creating enogh oil pressure to circulate oil, thereby damanging your engine. I've got a computerised auto in mine and it won't drive normally below 1200 although it will sometimes go down to 1100 for a bit in certain conditions. This could be more to do with the turbo spool speed though, which is about 1200.

Best suggestion I have for MPG is not to accelerate too slowly. Go reasonably to your chosen speed then keep it there as constantly as possible, preferably using cruise control. In my car car you can be driving along the flat at say 65mpg - if you ease down imperceptably on the pedal you won't notice any change in speed but the mpg can drop down to 40mpg. Tiny accelerations make a big difference.

On windy roads, roll/coast as much as you can by remaining in gear but using a teeny touch of pedal, and propel yourself by bursts of pedal at the right moments. This works much better in the Prius than the Passat mind 🙂

I second that. Accelerate as fast as you can, but keep the revs around 1-2K and the engine noise low (I tend to drive by engine note rather than revs, I've only ever driven one car with a rev counter). Sounds counter-intuitive but the best economy is a balance between speed and low revs.

Oh yeah and drop gears when you go down steeper hills. Not only do you get engine braking for extra safety (if you get the gear right it'll sit at the speed limit (or less if needs be) without any use of the brakes, it also pushes economy through the roof, though it won't sound like it from the engine noise. Basically, if you're having to brake, drop a gear instead. Makes a huge difference to economy and it saves you money on expensive brake pads.

Keeping in gear on the flat towards junctions helps (i.e. never coast, just let it roll), dumping the clutch only when you need to actually stop (or the car's about to stall)

it also pushes economy through the roof, though it won't sound like it from the engine noise.

I disagree. Any time you're off the pedal and in gear you're using no fuel. Changing down to a lower gear on steep hills will save your brakes but not fuel.

Another tip though is to slow down early. If you see lights coming up, lift off almost completely and start slowing down as early as you can*. No point in powering your car along fast and scrubbing it off at the last minute with the brakes.

* don't do this if there's anyone behind you though 🙂

Pretend you have no brake pedal, and a brick underneath the accelerator. Keep the revs as low as possible always. Just avoid it vibrating rough.

Annoy drivers behind you by taking ages to accelerate, and by coasting into roundabouts from half a mile.

Pump the tyres hard, and use something like Michelin Energy. (££ but they do make a difference - you know from cycling how much effort it takes to push the wrong tyres).

Team up with a 55MPH lorry on the motorways.

molgrips - Member
Well then don't. Dunno what you are talking about but I always change gear much less in a diesel on a windy country road than a petrol. When slowing into a corner you can stay in top and come out from 30mph if you want quite nicely - instead of needing 3rd.

You want to keep the revs as low as possible since the more time the fuel/air spends in the cylinder the more completely it'll burn, I think. Plus at higher revs you'll use less throttle - so youd burn the same amount of fuel in theory but in practice you'd be compressing more air you don't need, which is wasteful.

However if you drive at below say 1200 rpm you might not be creating enogh oil pressure to circulate oil, thereby damanging your engine. I've got a computerised auto in mine and it won't drive normally below 1200 although it will sometimes go down to 1100 for a bit in certain conditions. This could be more to do with the turbo spool speed though, which is about 1200.

Are you sure it's okay to drive like that all the time?
Somebody told me you need to give it the beans regularly, or it causes a build up of 'coke' in the engine somewhere and clogs up a filter or something.

Recently did a 200 mile motorway journey at night in a 2.0 diesel, 41mpg @ 84mph average according to on board trip.

Around town I struggle to get 30mpg, motorway @60-70, easy over 50mpg.

what Molgrips says is true, but its a bit of a hassle driving like that!

Id say your doing alright, although Vauxhall's are rubbish 😯

Are you sure it's okay to drive like that all the time?
Somebody told me you need to give it the beans regularly, or it causes a build up of 'coke' in the engine somewhere and clogs up a filter or something.

depends on the engine, some absolutely need a thrash every so often, as a decoke can be a very (£3000+) expensive procedure

Re engine braking. Aren't brake pads cheaper than clutches? I'd be inclined to use my brakes in preference to engine braking unless I were driving in hilly terrain.

Why does engine braking destroy your clutch?

service it every 6k miles, oil and air and oil filter, use an engine flush.

use the injector additive every 4k or so.

you have an older cruder diesel that will love the above treatment, tis a big car though.

Expensive diesel does help, but it works out more expensive than buying the additive yourself and putting it in.

You do need to thrash it periodically - known as the Italian Tuneup. Driving at low revs won't cause much of a problem as such - it's more short trips that are the problem I think. If you do 70mph for long trips that's usually ok.

Having said that a thrash every so often will still help. Our car's way smoother after caning it on the Autobahn a bit 🙂

Re tyres - it's not snake oil, the energy savers really do help. They are made with silica in the compound, a well known additive that changes the properties of the rubber. They are not simply harder rubber. It's the same stuff they put in higher end bike tyres - lowers rolling resistance and increases grip and wear. And that's another nice side-effect of energy savers: they last flippin ages. Mine on the Prius might be half worn ish after 40k miles!

To come to Germany I got snow tyres for the Passat which happen to be made from silica compound and they are easily 2-4mpg better than the old summer tyres all the time despite being snow tyres! Very impressed. Nokian WR G2 if you want to know.

TheArtistFormerlyKnownAsSTR - Member
Why does engine braking destroy your clutch?

Well in itself it doesn't but if you have to change down to get the revs up to provide engine braking it, errr, increases wear through additional gear changing. Doesn't it?

My comment was in response to 'drop gears... etc' intimating additional gear changes.

Even now I can picture my driving instructor banging on about braking then changing down to match rather than slowing down on the engine. Mind, that was so long ago I was learning to drive a traction engine 🙂

Engine braking as a technique is really for long hills where you might cook your brakes. It's about staying in a low gear and letting the engine race and help the brakes.

Letting the engine slow the car as you approach a junction say will help only because it takes a lot longer, so you'll be off the gas for more time and hence save more fuel.

I was told I was unsympathetic to the engine whilst doing a defensive driving course for work, as I was using the engine to slow. I was always told to change down in anticipation of slowing/ stopping so you can apply power quickly if needed and drive out of a situation. Apparently no, I should leave it until the car starts to judder, depress clutch and then brake! Made no odds to how I drove and given up and gone to auto now.

Maximising mpg is what too many people seem to be doing on my 'commute' - for gods sake, speed up!

I've been doing the same commute (car and/or motorbike) on and off for over 10 years now, and its taking longer and longer - but its not busier!

Rant over.

And Molgrips, whats the engine braking rubbish you are spouting, transmission = £2000 while brakes = £200, go figure which one to use.

Yeah I don't mess about like that when there's anyone else around! Constant speed for me. However not for everyone else it seems.. grr ...

And Molgrips, whats the engine braking rubbish you are spouting,

What you on about? I am not advocating trashing your transmission, am I?

I don't mean let the engine scream away at 8krpm. Just ride the engine braking if it's a really big steep alpine hill. That's always been the advice, and it's good too. you don't really have to worry about it in this country anyway, not with modern brakes.

@ SPud are/were you fuzz?

Can't work out do you mean they told you to drive with the engine leaping around then change down or to brake and then get in the appropriate gear?

@ Moly That's my impression of engine braking. Only on steep long hills and/or with added twisties. I was really saying changing gear just to introduce engine braking during normal driving could prove more costly than using brakes then changingd down ...

Perhaps but I'd be very surprised. How many transmissions fail nowadays because of tooth or synchro wear?

However it's not a very pleasant technique for you or your passengers.

Clutch not transmisson. Especially with added DMF changes often mandated by the manufacturers. 'Clutch' change now ~£400 - £500.

Give the car a good service, the correct oil, fresh air and fuel filters can all help fuel consumption considerably.

Change the tyres for energy savers but for gods sake don't pump them up any harder as suggested earlier, you will just wear them out unevenly and the smaller contact patch will increase the risk of skidding, especially if you have to stop hard.

Clear all the junk out of the boot, and don't be tempted to drive out of your way to save a penny a litre on fuel.

According to Mythbusters, use the windows in preference of air con below 45mph, above 45mph shut the windows and use the air con 🙂

If you get into the habit of letting the car slow down in gear up to roundabouts, be aware that as the engine revs drop towards idle the engine will start fuelling itself to avoid stalling, so a single block change from 5th to 2nd is better done early rather than just before the engine starts getting lumpy.

And don't sit there waiting for the car to warm up before driving, just get in and drive off and it'll be warm in 5 minutes rather than the 30 it'll take sitting on the drive 🙂

If you use engine braking properly combined with braking, there's no reason for it to be anything other than smooth.

i.e. slow the car with the brakes, then change down, brake some more and change down again if required, keeping it smooth. I don't brake with the engine at revs any higher than normal driving, say 2.5k revs. Just ramming down the gearbox to slow down isn't going to do the vehicle any favours.

Aircon? I followed the 'turn it off in the winter to save running the compressor' mantra and the seals dried out. To get to it meant dash out plus lots of work. Plenty £k's. Penny wise pound foolish.

I run my climax control all the time now. Bugger the fuel consumption.

Right, get yourself a pre burn catalyst (effectively a ring magnet) an install it round the fuel line as close to the pump as you can. This will align all the fuel molecules into the same orientation and gives a more complete fuel burn- cost about £90.
Next fit a K&N replacement air filter cartridge cost about £45.
Michelin Energy Tyres cost-about £30 per tyre extra over say Goodyear.
One in four tanks of posh diesel or Millers Diesel Addative, raises the Cetane rating of the diesel for better mileage/cleaner burn.
As you are doing,hit the next gear before your max torque so you are in top appropriate gear for road speed.
Oil-use the best you can afford/use in your engine, I use mobil 1 turbo diesel 0W-40.
I drive my 1998 VW Passat 110 hard and still never ever drop below 48mpg on a tank and on a run say to germany or Scotland get 65mpg minimum at motorway speeds.
Last driving tip is use your sense of acceleration and deceleration when driving and look well down the road so use of the brakes is minimum. On a motorway for instance by looking well ahead and planning you should hardly ever have to touch the brakes.
You may also be able to get an engine remap and chip for your engine for about $400 but remember some stuff can invalidate your insurance unless you declare it- ie performance enhancing. For me a replacement air filter to a permamnent filter is simply a method of reducing servicing costs and the pre burn ctatlyst to prevent the build up of soot in the engine.

boblo...just run it once a month 🙂

I was taught that if using engine braking to assist the brakes on a big hill, descend with the same amount of revs that you would be using when climbing the hill. The steeper the hill, the more revs you should have. So that'll pretty much be the redline in my works van then 😉

[i]Right, get yourself a pre burn catalyst (effectively a ring magnet) an install it round the fuel line as close to the pump as you can. This will align all the fuel molecules into the same orientation and gives a more complete fuel burn- cost about £90.[/i]

I'm always sceptical of this one...if it makes a difference why don't the manufacturers fit them? And £90 for a magnet?!

No, not police. I was apparently, letting the car slow and as it did/ does change down to what I would say 'match' my speed and gear, so there was some engine braking. The 'right' way apparently is brake and only when needed (i.e. it's bloody obvious the gear is too high and you get that about to stall feeling) change down. Hard to get across in words I think. But I argued the point and was told I was too hard on the engine etc.

Its already been mentioned but keep the engine in gear as long as possible, this uses less fuel, if you dip the clutch or coast in neutral modern cars 'idle' using more fuel.
If you get to a set of traffic lights you know that have turned red, turn the engine off and wait for your turn before starting the engine. You use a lot of fuel in a modern car sat at traffic lights idle-ing.
We had to do a smart driving course in a new stop start BMW. Great technology and the difference with the system on and off is about 10mpg about town. It was also mentioned about energy saving tyres, keeping windows shut at high speed, aerodynamics etc. I even got a lovely certificate saying I had improved my mpg by 20% and would save the council £300 quid over the year. I then drove back in my works Landrover with mud terrain tyres a full roof rack and the back full of tools 🙄

To prevent build up of soot in your engine just blank the EGR valve off, most places do them as a performance upgrade.

Use the torque Luke.

Too many of us use the wrong gears in town.Look back on todays drive and you will probably find that you went into 4th or 5th in a 30 zone a few times.now check what speed your car will go in 4th and ask if those gears were necessary!

I slow the car down quite often going through the gears.a wee blip here and there.still on the same engine,clutch and gearbox and its 209k. Its the salt on the roads thats seen the death of my car,not the way its driven or the quality of the car itself.

I would guess that for around £40 you could change the oil to a thinnner type for colder use in the winter months to help with how efficient the engine could be.did we not talk about this last week?

Very rare that I get more than 40mpg from our Mondeo 2.2 TDCI. Mostly country roads though.
On a run 45mpg is more normal.

Love the torque that diesels have and the MPG is far less effected by heavy right foot driving than petrols.
I can drive much harder and it wont go less than 35mpg, where as in a similar petrol the MPG dropped by 50% or more.

Just like to share that i hit the magical 67mpg mark in my 1.8tdci focus today 😀 Thats driving from liverpool to london and back, best thing was, only costed £41 in diesel when i got back Was sat between 1750 and 2krpm most of the time)!

I agree, if you're doing motorway miles, ease the accelerator to go quicker, you'll notice that you'll accelerate quicker and use less fuel if you slowly push the pedal down than just planting it.

Oh, and don't be fraid of sitting behind a lorry doing 60/65, as they help a lot when it comes to using less fuel! (With them pulling you along and that) As for short journeys, use the bike 😉 Diesels aren't great for quick shooting to the shops, they tend to use more fuel than petrols if you're using the car for short journeys (couple of miles) as they do not heat up.

Wasn't the 2.2 diesel in the Omega borrowed from BMW.

I reset the computer on my 'company' Ford Focus 1.8 turbo diesel today and achieved an average of 18.6 mpg on mainly semi-rural roads. That did include about 2 hours with the engine running and not driving anywhere!

When actually driving, it is very low 20's to the gallon.

Our a3 2.0 tdi went back after 3.5 years of mainly town driving. Computer over that time said 34.9 mpg. Brim to Brim said 33. This was mainly driven by my wife. In that time it had a few major issues fixed under warranty.

Oil consumption was 1L per 8,000 miles.

She's since replaced it with a petrol bmw (arrrgh!) 116i (2.0L) that does 35 mpg on the same routes. The stop start effecient thingy just suits her (revvy) driving. She doesnt know or care to be honest.

On long trips it probably less eco but we genearlly dont use it for long ones (no boot etc).

So in retrospect - buy a diesel - change driving style, expect more repair bills and less stops at the pumps. Which is more "economical" depends on you, your miles, what breaks etc.

My point is that YOU need to change - not the fuel / car. Adapt to what you've bought.

I was told I was unsympathetic to the engine whilst doing a defensive driving course for work, as I was using the engine to slow. I was always told to change down in anticipation of slowing/ stopping so you can apply power quickly if needed and drive out of a situation. Apparently no, I should leave it until the car starts to judder, depress clutch and then brake!

Well, thanks for all the advice, some very interesting responses.
I'll be trying out a few of the techniques mentioned and see if the MPG changes over the next few tankfulls.

Inbred456 - Member

Wasn't the 2.2 diesel in the Omega borrowed from BMW.

No, that was the 2.5 6 cylinder, which apparently came with loads of issues as standard.
This one is the old GM/Isuzu unit.

molgrips - Member

Quite like the gentle nature of the power, but sick of changing gear all the time!

Well then don't. Dunno what you are talking about but I always change gear much less in a diesel on a windy country road than a petrol.

As to the suggestions of spending more cash on tyres, air filters etc: I recently bought the car from a colleague's husband for £350 - it's done 250,000 miles and I'd like to keep the expenditure down as much as possible - I've changed the oil and filter and will look into energy saver tyres and a flash air filter when it comes time to replace them.
If it does go bang at least I'll get £200.00 from the scrap man for it 😀

Right, get yourself a pre burn catalyst (effectively a ring magnet) an install it round the fuel line as close to the pump as you can. This will align all the fuel molecules into the same orientation and gives a more complete fuel burn- cost about £90.

Inbred456 - Member
Wasn't the 2.2 diesel in the Omega borrowed from BMW.

As above, that was the 2.5. In Vauxhall tune, 137 BHP (IIRC) which was a tad underpowered for the weight of the car. Once up to speed tho, she did waft along. I had mine from new to 140k, no issues at all. Just disks and pads and servicing. Nothing else. Fantastic car.

Right, get yourself a pre burn catalyst (effectively a ring magnet) an install it round the fuel line as close to the pump as you can. This will align all the fuel molecules into the same orientation and gives a more complete fuel burn- cost about £90.

Serious?

RS - right.. well there are good and bad diesels, can't say which kind you have but the distinction was even greater ten years back 🙂

My point is that YOU need to change - not the fuel / car.

Both. Diesels are more efficent, this is a fact. You're not comparing like with like by comparing a petrol with stop start to a diesel without.

As for tyres, because energy savers last far longer than normal tyres, they are actually cheaper ime, and then there's fuel saving on top. They might last 50k miles or more. However if you have an old Vauxhall with over 200k miles on it's probably not worth it as the car itself may not last that long 🙂

I'd be cautious about sticking too many additives in diesel as I've read that it can strip a coating off the engine. Using a premium diesel every few tanks has the same beneficial properties but also the essential lubricants. I'm no expert, its just what I've read.

The additive that works is called 2-ethylhexyl-nitrate or 2-EHN, and it makes the fuel burn more readily. It's got a very distinctive smell, and you can smell it in all those 'power booster' things and also the premium diesel.

Rusty, a couple of things. I drive an underpowered diesel-fueled lump of a car. Recently it started asking for 35mpg on a motorway and country roads. Too much for my liking.
I gave it some diesel, added Diptane (check my Pug post or google it) and after a few miles to mix it in the tank we went for a drive. Some 25 miles of twisty Irish country roads (R108 Drogheda to Swords), 3rd gear and over 3k rpm all the way. She cried at first, farted and coughed, then started singing. 3-3.5k rpm and she went like a stink. The consumption hasn't gone up (and the road is properly twisty and hilly) compare to the flat and boring motorway, the car pulls better since.
The tyres - when I needed a set some 2 years ago I chose Milleniums, cheap and Avon-made, good enough for me. We happily spent 18 months driving all over the place including a blast from London to Poland with the boot overspilling. Several trips between London and Dublin followed. The front wore but the rear didn't. As my Pug is FWD it probably affects the tyre wear. I considered Michelin Energy Savers but they were costly and... at least in the opinion of the 406 drivers' forum, they give little grip in wet or slippery. So I found Uniroyals - proper rain tyres. The grip is immense, the ride supple, the noise - you have a radio for it.
Bottomline - just drive the thing, fix what needs fixing, change as and when required. Oil, filters, air pressure in the tyres, they all make a difference. Driving style? Not unless you're a moron.
Enjoy!

I considered Michelin Energy Savers but they were costly and... at least in the opinion of the 406 drivers' forum, they give little grip in wet or slippery

They are apprently much better now with the newer versions.

We have some from a year or two ago (not sure if they are the new or not, they might be) on the Prius and I have certainly not noticed any issues.

Top tip seems to be drive really slowly 🙂 This morning I had very little fuel and the petrol station at home is not convenient, but the one at work is. Stayed behind a lorry the whole way at 47mph and got 65mpg!

Ha! Moly with your green credentials, you should have been in that lorry and that would have neutralised your incremental carbon footprint for the day. Shame on you 😉 🙂

I should have been on my bike! I've attempted every morning this week to ride in, and failed each time. First four days I actually put on cycling gear only to have to take it off again. Usually a combination of too much faffing, ill-preparedness and extreme tiredness leading to over-sleeping or morning lethargy, however last night I was just putting a bit more air in the tyres and blew a valve, spare tube had multiple punctures 🙁

Interesting though that I'd normally get 51-52mpg on the same run, so going almost 50% faster on average only results in a 15-20% drop in mpg.

Drive a bit slower, 60mph will make a decent difference and unless you are going miles & miles it won't take that much longer. You'll probably arrive more relaxed too.
Look further ahead - no point accelerating if you can see a queue/roundabout/sharp bend ahead.
Slow down earlier on the approach to roundabouts/traffic lights. Let the car slow down, rather than driving at the same speed up to the obstacle & then braking hard.

I have started doing this on my commute and fuel consumption has gone up loads. Managed this on the way home from work last week (60 miles):

[IMG] [/IMG]

If you could run the engines at high revs constantly, diesel and petrol engines would give you similar power and efficiency. However, because you spend more of your time driving at less-than-optimal revs, that is where the diesel gives you the better performance.

Has anyone had their diesel engine re-mapped? Interested in experiences.

If you could run the engines at high revs constantly, diesel and petrol engines would give you similar power and efficiency

Not entirely sure this is true. Diesel has more energy in it per litre, and also diesels have much higher compression ratios which results in greater thermodynamic efficiency.

They would be closer if you ran at high load, because petrol engines run more efficiently with the throttle wide open. It's inefficent to have to suck air through a tiny hole.

Incidentally VW's FSI and TSI engines are direct-injection petrol engines meaning they don't use a throttle (as much, if at all) - they slow the engine down by simply injecting only a little bit of fuel directly into the cylinder and cleverly making sure it ends up near the spark so it ignites.

Has anyone had their diesel engine re-mapped? Interested in experiences.

Had my Leon 2.0 TDI (PD) remapped a couple of years ago. Fuel consumption is roughly the same (if not slightly improved) but a lot more responsive in gear. I think the key is that you can accelerate up to speed more quickly.

As mentioned, a few MPH slower can make a significant difference to the MPG as can the usual suggestions of turning of the air-con, ensuring your tyre pressures are correct (or even a few PSI over), removing the roof-rack, and emptying your boot of unnecessary junk.

It gets to be abit of a game. More interesting than speeding tickets. -I've done with those.
I'd vouch for the lorry tracking. Only on quiet motorway in bone dry conditions mind. If you are sensitive to it, you can feel the draft from about three lorry lengths behind. I averaged 73MPG on a 350 mile round trip last year. Used about 1/3 of a tank full. Moderate gap, about "one chevron", so I wasn't being too loony.

Old Focus 1.9 TDCI with 150K miles.

you can feel the draft from about three lorry lengths behind

I'd like to point out I was NOT slipstreaming the lorry, merely not overtaking it thereby giving myself an excuse to drive at 47mph.

molgrips - I use the Nokian WRG2 over here. Simply the best tyre I have used. Perfect for UK winter as works very well in the wet and dry as well as snow and ice. I noticed the improved MPG too over Xmas.

Obviously they will be swapped over with my summer tyres soon but keeping them on for the time being. Simply amazing feedback on wet and slimy roads.