How to drive a diesel most economically?

My mates company got a police driving expert in to coach the fleet of company car drivers in order to save fuel. Turns out they were driving to slowly and changing up to early. The trick apparently is to maintain the highest gear you can while at peak torque and not to worry if they cant get it in sixth gear. 5th gear at slighly higher revs was in most cases more efficient. So they tried this and to their amazement he was right, diesel costs have come down significantly. So put your foot down!

but I also drive modern diesels as company pool cars and they don’t like it. If you don’t change down then they increase speed thinking a stall is coming up – they can fight quite hard against the brakes if you do this

I will do 5th to 3rd (or 2nd) as the engine gets near idle and before it starts pulling, I just meant if you’ve stayed on the throttle a bit too long, there is no point using dropping additional gears to scrub off speed without the brakes “

I wasn’t criticising you – just pointing out that, to my dismay, the modern car takes over and does not allow me to drive in the manner I have done for decades (and generally managed to better published and friends’ MPG figures).

I could get 55 mpg out of a petrol Renault 6 back in the day and not be holding people up.

Most important is to keep up a steady pace, stay off the brakes and the throttle. To get the best mpg you need to up your concentration levels above what is normsl for many. The other big benefit is the more you drive like this the more relaxing your journies can feel.

Accelerating I say short shift BUT keep the turbo on boost all the time. For my mondeo that means keeping the rev counter between 2000 and 3000ish.

Put it in neutral when going down a hill (only when safe obviously) using gravity not fuel.

Rubbish.
A modern engine management system uses no fuel on overrun. That’s why the mpg readout goes sky high as it’s literally running on fresh air.
Idling in neutral uses fuel to keep the engine running whereas leaving it in gear and off the throttle makes the car ‘drive/turn’ the engine.

Rubbish.
A modern engine management system uses no fuel on overrun. That’s why the mpg readout goes sky high as it’s literally running on fresh air.
Idling in neutral uses fuel to keep the engine running whereas leaving it in gear and off the throttle makes the car ‘drive/turn’ the engine.

whilst that’s true, you’re using the kinetic energy of the car to drive the engine at 2,500rpm (say) instead of using the energy in your fuel to drive it at 700rpm. leaving it in gear uses more energy. If you’re stopping at the bottom of the hill anyway, then that’s fine, however if you’re just driving up the other side of the hill, you may find dropping to neutral is best, purely from an economy perspective.

there was a report a few years back that proved for a given car (polo diesel rings a bell) the most economical way of getting up to speed was to floor the throttle in each gear, up to 2,000 rpm, then shift up. That way you were in a lower gear for less time, which increased economy.

The whole revs thing is engine dependant. on most engine, the lower the revs, the more economical it is. On some petrol engines, with high cams, you can end up pumping unburned fuel into the exhaust this way, which isn’t good, but this isn’t possible to do on a diesel engine.

lower speed helps hugely, that (whilst you’re still in top gear) and staying off the brakes are probably the best things you can do

I was driving the works car at the weekend, a big 4×4 diesel. However it has both an Instant and Average mpg readout on the dash so I amused myself by seeing how high I could get the two figures throughout the journey. (38.8 average, mix of motorway and urban).

The comments above about being gentle and reading the road really apply, it’s amazing how frugal it can be if you can stay off the brakes, coast in gear and be smooth on the power. Equally it’s amazing how low the numbers go if you boot it…

The whole subject depends on the engine your driving and the area you drive in.

I have 2 diesel cars, one a Ford, the other a VAG. The Ford its more comfortable at lower revs and therefore more economical, the VAG needs more revs. The Ford will go along the motorway at 70mph in 6th at less than 1750rpm, the VAG will be nearer 2500 rpm.

Making sure your tyres are at the right pressure helps alot too.

My favoured technique is to give bursts of acceleration when you need them and then coast the rest of the time. So acceleration is moderate, not too slow, and when I say ‘coast’ I mean just run with a tiny bit of throttle – just touching the pedal. If you lift off completely you are not using the brakes but the engine is braking by compressing air.

You can’t do this in traffic though – it’s far too annoying for those around you. On motorways I use the cruise and sit at a constant speed. Creeping gradual acceleration and deceleration kill your mpg. You can ease down on the pedal so slightly that you can’t even feel any acceleration and yet your instantaneous mpg will drop by 50%.

I wouldn’t floor it in low gears – the car will over-fuel and more will be wasted – just accelerate somewhat briskly.

Idling in neutral uses fuel to keep the engine running whereas leaving it in gear and off the throttle makes the car ‘drive/turn’ the engine.

Yes BUT if you leave the car in gear you’ll slow down far more quickly so you’ll have to drive for longer using fuel. If you put it in neutral you can coast for ages. When I’ve tried this in the past on repeatable identical short journeys it made a significant difference putting it in neutral well in advance of a roundabout and coasting than just lifting off a shorter distance before. I could get the same effect by decelerating very gently under very light throttle though.

the down side of never accelerating hard is, if you have a DPF, it will never clean out and can lead to an expensive trip to the garage. So once in a while u need to push these cars hard and look for the cloud of black smoke out back.

keep tyres corerectly inflated. As it’s just got cold, u may find u need a bit more air in them.

don’t carry anything u don’t need.

Peak torque is where the engine is combusting most efficiently, so aim for there. If you stay below where the turbo begins to work, over time the soot build up clogs the turbo and then you have big bills replacing it. (It was interesting explaining this to a chap that drove back & forth from Bucks to Cornwall on the motorway at 55 in 6th trying to get the most economy from his car to then get a bill for far more than he saved). To save fuel, don’t do short journeys (especially with a cold engine)accelerate gently, don’t labour the engine, and never brake (if you can help it, as it just wastes the energy you have created by burning the fuel in the 1st place). Look ahead and anticipate.
I wouldn’t go mad just to get another 1 or 2 mpg. Life is too short.

“I wouldn’t go mad just to get another 1 or 2 mpg. Life is too short.”

Exactly, on the motorway say doing 55-60mph my car does about 53mpg. Same journey doing 85mph I get 50mpg!

Peak torque is where the engine is combusting most efficiently, so aim for there.

that’s not strictly true on engines which can alter the amount of air going into the engine. for instance, a turbo or a set of trick cams can allow peak torque quite high up in the rev range, but driving like this would use a lot of fuel.

Not sure if it applies to the PSA engine but in a 1.9TDI (x2) I upshifted just before the engine would labour – ie in a petrol where normally you’d be just in 2nd on a straight I’d already in 4th etc etc. I beat the manufacturers official figures doing this!

Basically ride the torque.

Yes BUT if you leave the car in gear you’ll slow down far more quickly so you’ll have to drive for longer using fuel. If you put it in neutral you can coast for ages. When I’ve tried this in the past on repeatable identical short journeys it made a significant difference putting it in neutral well in advance of a roundabout and coasting than just lifting off a shorter distance before. I could get the same effect by decelerating very gently under very light throttle though.

You’re missing the point of a diesel. Short journeys do it no good and it’ll never return it’s full economy potential.
I have gotten more than a cars book economy figure in every car I’ve driven, repeatedly.
I rarely leave an engine idling. It’s wasted fuel. You’re using the engine for no reason.
Just pick a higher gear if you find you’re slowing down too much.
A diesel engine is least efficient whilst accelerating, in fact it’s quite inefficient. It is most economical when steady cruising at peak torque, not peak power.

The way to get the highest possible fuel economy return is to accelerate briskly, 1/2 to 3/4 throttle, then cruise at whatever RPM the engine produces peak torque, then gradually slow down on overrun if the need arises, to adjust to changing conditions, then repeat etc.
This is from Peugeot motor co technical training department.
As they make a lot of the diesel engines on the roads at the moment I think they have a fair idea of what they’re doing. (Peugeot, Citroen, Ford, Volvo, Mazda, Suzuki, Jaguar, Mini all use Peugeot diesels)

Unfortunately the nature of a diesel engine then requires a damn good thrashing once in a while to decoke the exhaust etc to release maximum efficiency.

The way to get the highest possible fuel economy return is to accelerate briskly, 1/2 to 3/4 throttle, then cruise at whatever RPM the engine produces peak torque, then gradually slow down on overrun if the need arises, to adjust to changing conditions, then repeat etc.

that’s just not a true statement. that may be the best way to drive *a* car, but its not the most economical way to drive all cars. even taking a pugeot – 206 gti (as its the first i could think of) – peak torque is at 4250rpm. Using that engine speed will not be the most efficient way to drive that car.

I rarely leave an engine idling. It’s wasted fuel. You’re using the engine for no reason.
Just pick a higher gear if you find you’re slowing down too much.

even in top gear, down a hill, a car will decelerate. this uses energy. down the same hill, in neutral, a car will decelerate less. to get you back up to the original speed, will use more fuel than if you had been idling down the whole hill. essentially, you’re wasting the energy required to spin the engine & gearbox at 2500rpm instead of 700rpm. its a minimal difference but it is a difference.

if you turned off the engine and coasted you’d be doing even better, but you probably would be sacrificing too much safety/convinience in doing so

FWIW the cruise control on the mighty Mondeo is much more economical than I am, especially on undulating terrain.

It is more sensitive to minor changes in speed so when going downhill it eases off earlier than I would and similarly when going uphill the modulation is superb.

It’s a joy to take one’s foot off the pedal and feel the engine sorting it out on its own.

the down side of never accelerating hard is, if you have a DPF, it will never clean out

It will. Short trips are what kills DPFs. Accelerating hard is how you clean it out after it’s clogged through short trip usage.

You’re missing the point of a diesel. Short journeys do it no good and it’ll never return it’s full economy potential.

Er, I didn’t select a short journey to maximise efficency. I was doing a short journey because the two places between which I was travelling were not very far apart 🙄 and yes, too far to walk in a reasonable time and no bike was available.

Just pick a higher gear if you find you’re slowing down too much.

Well dur. I only have 6 to choose from, I was already in max.

Almost every mile I drive I am trying to maximise fuel economy, so I’ve picked up a pretty good understanding of my cars.

The way to get the highest possible fuel economy return is to accelerate briskly, 1/2 to 3/4 throttle, then cruise at whatever RPM the engine produces peak torque, then gradually slow down on overrun if the need arises

That’s exactly what I said 🙂

that’s just not a true statement. that may be the best way to drive *a* car, but its not the most economical way to drive all cars. even taking a pugeot – 206 gti (as its the first i could think of) – peak torque is at 4250rpm. Using that engine speed will not be the most efficient way to drive that car.

even in top gear, down a hill, a car will decelerate. this uses energy. down the same hill, in neutral, a car will decelerate less. to get you back up to the original speed, will use more fuel than if you had been idling down the whole hill. essentially, you’re wasting the energy required to spin the engine & gearbox at 2500rpm instead of 700rpm. its a minimal difference but it is a difference.

if you turned off the engine and coasted you’d be doing even better, but you probably would be sacrificing too much safety/convinience in doing so

Aside from that a 206 GTi is a petrol engine and a different kettle of fish entirely, the same principal applies.
I have a phase 1 106 Rallye and that is most efficient, ie returns 48mpg, on a run at 4850 ish rpm in 5th gear.

Regardless of how much speed you lose on a downhill, neglible really if any kind of gradient other than a gentle slope.
My own diesel actually accelerates down a reasonable hill in gear.
Just pick the gear that does that and then hold it back lightly on the brakes if needs be.
The short acceleration burst needed to regain cruising speed outweighs the amount injected to just keep the engine running.
For a gentle slope just back off the throttle a bit.

Aside from that a 206 GTi is a petrol engine and a different kettle of fish entirely, the same principal applies.
I have a phase 1 106 Rallye and that is most efficient, ie returns 48mpg, on a run at 4850 ish rpm in 5th gear.

80mph in a car is never the most economical it could be driven at. the force required to push a car forwards at 80mph is somewhere around double the force (per mile) required to push a car forwards at 55mph, due to wind resistance being an exponential force. Whilst the engine may be operating at its most efficient speed, the inefficiency in moving a car that fast overrides that. In addition, a traditional petrol engine is at its most efficient at WOT to avoid throttle losses – running a nearly closed throttle at high revs will maximise both pumping and throttle losses.

Regardless of how much speed you lose on a downhill, neglible really if any kind of gradient other than a gentle slope.
My own diesel actually accelerates down a reasonable hill in gear.
Just pick the gear that does that and then hold it back lightly on the brakes if needs be.
The short acceleration burst needed to regain cruising speed outweighs the amount injected to just keep the engine running.
For a gentle slope just back off the throttle a bit.

as I said before, if you need to use the brakes, then yes, leave the car in gear. However, if at the bottom of the hill you need to accelerate, you’d have been better off going faster with the car out of gear.

essentially, the engine is turning faster if you leave your car in gear. the energy to make the engine turn faster has to come from somewhere. As there is no free energy, it comes from burning fuel. whilst you’re not burning fuel at the time, the energy comes from somewhere else-the kinetic energy in the road speed of the vehicle. to regain this energy, you press the throttle for a short amount of time at the bottom of the hill, however this uses *more* power than idling the engine for the whole way down the hill.

On the flip side, the vehicle travelling faster will actually have used some additional power by the virtue of increased aerodynamic drag, so you’d have to build that into the equasion as well. but you’d also get home quicker, I’m not sure how you quantify that

incidentally, there’s a load of info here

http://en.wikipedia.org/wiki/Fuel_economy-maximizing_behaviors

of gear (manual transmissions)Engine efficiency varies with speed and torque, as can be seen in a plot of brake specific fuel consumption.[20] The optimum efficiency point is around 1750 rpm, and 90% of maximum torque at that speed, for this turbo-diesel engine. For driving at a steady speed, one cannot choose any operating point for the engine—rather there is a specific amount of power needed to maintain the chosen speed. A manual transmission lets the driver choose between several points along the curve. In the turbo diesel example, one can see that too low a gear will move the engine into a high-rpm, low-torque region in which the efficiency drops off rapidly, and thus best efficiency is achieved near the higher gear.[20] In a gasoline engine, efficiency typically drops off more rapidly than in a diesel because of throttling losses, and the trend discussed here is even more dramatic.[21] Because cruising at an efficient speed uses much less than the maximum power of the engine, the optimum operating point for cruising at low power is typically at very low engine speed, around or below 1000 rpm. This is far lower than the above mentioned 1750 rpm. This explains the usefulness of very high “overdrive” gears for highway cruising. For instance, a small car might need only 10–15 horsepower (7.5–11 kW) to cruise at 60 mph (97 km/h). It is likely to be geared for 2500 rpm or so at that speed, yet for maximum economy the engine should be running at about 1000 rpm to generate that power as efficiently as possible for that engine (although the actual figures will vary by engine and vehicle).

The alternative to acceleration and braking is coasting, i.e. gliding along without propulsion. Coasting is an efficient means of slowing down, because kinetic energy is dissipated as aerodynamic drag and rolling resistance, which must always be overcome by the vehicle during travel. When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars’ engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required