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n Usenet, many people believe I
judge a car solely by its fuel economy, hence my preference to diesel
engines. It’s a bit more complicated
than this, but I do not doubt that an individual car’s fuel economy can make or
break a decision. But it is not the
primary reason for model choice, otherwise we would not have changed the Mondeo TD for the Ka. We certainly not have bought the Honda
Accord, but consumption was a major reason why I bought the Saab 9-3.
Even then, the Euromix figure
cannot tell the whole story. The Urban
and Extra Urban figures, or the old style Urban, Constant 56 and Constant 75 mph
figures can give you at best an indication
of how efficient a car is over different types of 
driving.
For example, using the old style
figures, a car with a low Urban economy figure, but high Constant 56 and
Constant 75 figures is perhaps somewhat heavy, long in the gearing department
and with aerodynamic efficiency. An
example is the Mondeo TD: it’s urban
economy figure was somewhere in the region of 38 mpg, but at a constant 56 mph,
it managed 64 miles per gallon! At 75
mph it managed around 45 to the gallon.
The Cinquecento was
somewhere in the region of 38 / 60 / 45 too.
Before then the Fiesta diesel
was 40 / 70 / 50, or close enough.
The most economical driving
technique is to avoid unnecessary changes in speed. When you accelerate, the engine has to
provide more power, which in turn burns more fuel. Deceleration in itself isn’t a bad thing, of
course, but you need to accelerate back up to speed, which burns more fuel.
Of course, acceleration and
deceleration is a part and parcel of driving.
There are ways and means to avoid wasting fuel through unnecessary
acceleration, at least, when you’re not trying to have fun. To many people, these techniques feel very
slow, but in reality they make a tiny difference to the average speed of the
journey, but can make a significant difference to fuel economy, as well as
making a more comfortable drive for anybody else in the car, saving wear and
tear on various mechanical bits and pieces, and - perhaps – making you a safer
driver.
In essence, the aim is to be as smooth as you possibly
can and to effectively anticipate when you will need to change speed. Smooth doesn’t always mean gentle
acceleration, but it means applying the power with some sensitivity, i.e.
“rolling on” rather than “snapping open” the power and in your use of the
brakes.
Acceleration Sense is a very useful
skill that once learnt in one vehicle, may be applied to almost any vehicle,
probably even tanks. Acceleration Sense
is the ability to judge how much power to apply now to successfully anticipate
some future action. It may mean
accelerating on the flat to help the car climb a hill or lifting off with
several hundred yards to go because of a roundabout you are approaching.
It’s quite possible to lift off
much sooner than you have to use the brakes, for example, coming up to a
traffic light, junction or roundabout.
At roundabouts, where possible use forward planning and observation so
that you arrive at the roundabout in the right gear, at the right speed, at the
right time to avoid having to stop.
General
Economy top
- Use the gears properly. Sensible, careful use of the gears can
also save fuel - and this doesn’t always mean being in the highest
possible gear for that speed. Dropping
the car into fifth gear at 35 may not be all that economical, since the
engine may be labouring (which is also bad news for the engine too). Similarly, just because the car will
happily cruise at 50 in top gear, doesn’t mean that whenever you reach 50
you should be in that gear. You
might want fourth when going up a hill, since the engine struggles in
top. I discuss gears and the
effects on performance here.
- Consider if you really need to overtake the slower car in front. Is there a section of dual
carriageway? Is the road too
busy? Are you really finding
55 mph too slow? How much time
would you save if you could drive at sixty rather than fifty five? The acceleration required when
overtaking uses fuel, but some drivers make many “run ups” before finally
overtaking. The “Run up” overtaking
technique is discussed here.
- If at all possible, travel when traffic is
light. This is what the Government want
us to do at least but of course it often simply does seem possible. Heavy, stop go traffic uses up lots of
fuel. An engine idling when the car
is stationary is getting zero miles to the gallon. Moving off wastes fuel: if you can keep
the car rolling, do so. But by this
I don’t mean, slow the car down to 15 mph when you’re a quarter mile from
the lights... I touch on driving in
quieter times on my average speed page.
- It’s also possible to save quite a bit of fuel when
in hilly terrain. Obviously, you
need to be in the right gear for the gradient, your car, and the speed at
which you’re travelling. Changing
down a gear or two might feel like it’s going to use up more fuel, but a
labouring engine is both inefficient and potentially causes long term
damage. When going down a hill, the
advice is to accelerate the car taking advantage of gravity, but on the
way up, lift off the power and let it decelerate. This isn’t easy if you want to stick to
the speed limit or are using cruise control, for
example. For a hill that’s not too
steep, giving it a little bit more power (changing down if you need to)
just before you reach the base of the hill, to start the car accelerating,
then holding on to that extra power, allows you to climb the hill
reasonably easily. I see too many
people only giving it more power when the speed starts to drop off, or
changing down, and losing all of their speed. If you have cruise control, manually increasing the throttle just
before you reach the bottom of the hill can save fuel on the way up.
- When you’re cruising, do just that. Maintain
as close to a constant speed as you can, rolling on or off the
power as you need to. On a busy
motorway or dual carriageway (the M25 is illustrated here),
consider cruising at 56 rather than trying to maintain 70 (or more), in
the inside lane. All too often,
trying to maintain higher speeds results in you having to change speeds
quite frequently, which means you’re burning up a lot more fuel for a tiny
increase in average speed. In the
inside lane, you may occasionally have to ease off to help somebody emerge
onto the motorway, but you can usually simply cruise along with the HGVs, something I now call “HGV Pace.” When
the motorway or dual carriageway is quieter, you can cruise at higher
speeds without having to slow down as often. Using cruise control can be a great help here! - Keep the car well maintained. A dirty air filter and underinflated tyres
make a significant dent in fuel consumption.
- Don’t keep the roof rack on if you don’t need it.
- Don’t carry stuff that you don’t need.
- Open windows and air conditioning systems use more fuel, either
through increased aerodynamic drag or sapping power from the engine respectively. For best economy, use the ventilation
system as best you can. For
those cars with air conditioning, use this when you need to. I’d not particularly advocate cycling
the system on and off every ten minutes or so because whilst this can save
fuel, it wastes driver time.
Instead, setting the heater control to be somewhere between cold
and hot (system application varies between cars). In the Ka and in warm but not hot
weather, use of the air conditioning reduces fuel efficiency by 5%. When it gets hotter, and when you spend
time in cities, the penalty is more than doubled! - If you’re not going to overtake, increase the
distance between yourself and the traffic in front illustrated so
as to maximise the buffer. With an
extended gap, you can ease up when they brake (the lead car turning
off, for example) rather than having to brake with them. It also, of course, increases the time
you have to react in certain emergency situations.
- Cold starts use a lot of petrol (diesels are
not so adversely affected). On
cars with a choke, use it as little as possible. For fuel injected petrol cars, there’s
little you can do besides avoiding cold running.
It’s also good practice to ensure that the engine is properly warmed
up on every trip. This makes sure
that the oil reaches all parts of the engine, the catalyst works, and all
water vapour is blown out of the exhaust.
- Learn your car.
Learn where it performs best, find out the recommended gear / speed
figures, and check your fuel consumption over all types of your driving (not
just after a long run).
Turbocharged
Engine Economy Tips
I love turbochargers!
These tips apply equally well to a turbodiesel or a turbopetrol engine.
- Learn where the turbocharger starts to kick
in. For a direct injection
turbodiesel (something like this) this is usually around 1,500 rpm. You also need to know at what engine
speed it’s really blowing, typically a few hundred revs higher. Use these engine speeds to assess what
gear to be in when tackling a hill; it is more economical to ascend a
gradient with the engine at a higher speed than trying to work the
turbocharger harder at a lower engine speed.
- Learn to feather the throttle when changing
speeds at a cruise, so as to not use too much turbocharger boost. A little boost at a cruise is often a
good thing, since it improves efficiency.
Too much boost wastes fuel.
- Depending on the car, learning where the “turbo
spot” is and then cruising at this speed or slower. Those cars with a variable geometry
turbocharger, you’re out of luck here; your turbocharger is already ready
and waiting!
- Don’t chase the power you get at high revs as a
matter of course. With a
turbodiesel, you have access to most of the acceleration without using
much more than half maximum engine revs.
Many turbocharged petrol cars are broadly similar.
An
Example
If you’re still reading, by now
you can see that saving fuel isn’t always about driving slower. As an example, an ex-colleague had an early Golf V5.
He always complained about his fuel economy, which was generally 27 - 30
mpg. I said that this wasn’t too bad,
given that the Euromix figure for the car was 30·4 mpg, and that it was
reasonably new.
Then he took me out in it. The man cannot maintain a constant
speed. Not at all. On the motorway, we’re either accelerating
through 75 on our way to 80, or decelerating through 75 on our way to 70. On a trip to London Heathrow from
My ex-colleague, of course, did
not believe that his car could achieve such an economy figure, then assumed
that I had driven back at 40 mph all the way.
It’s all about technique.
Driving For Economy: DervMan’s “e-max” Technique
It is fair to say that for most of
my driving career, and at least until late 2001, I’ve had to pay very close
attention to the costs. Whilst to some
people, a saving of 15% in fuel costs might not amount to much within the scope
of their salary, the difference between 38 mpg and 44 mpg over one year and
12,000 miles is approximately £150 (fuel
cost at an assumed 75p per litre).
That doesn’t sound like much, but when you are struggling with money,
that £150 you save is important: it pays for the road tax!
If you could get free road tax a
year, would you? By driving with economy
in mind, I could squeeze at least another ten percent out of every gallon of
fuel I used.
I think that everybody should be
poor, so that they can remember and sympathise!
If you’d like to argue that people shouldn’t run a car when they are
poor, feel free to contact me, or bring it up on
Usenet.
For the Fiesta Ghia, Danielle, I usually returned around 42 mpg. That’s not too bad, but there were times when
I was finding it especially lean. So I
decided to limit my maximum speed in the gears, in a fairly rigid manner. I used self-imposed speed / gear limit, which
I calculated in a simple manner of that gear multiplied by ten, then adding a
further 10 for the top gear. I called it
“e-max” (not having a good imagination!) and using this technique with
Danielle, I saw another ten percent squeezed from every gallon; economy figures
went up to 45 or 46 mpg. Danielle’s
economy was hampered by a four speed gearbox and try as I might, I was only
able to squeeze just over 46 mpg. It was
also difficult to drive to work at 50 rather than 60! For Danielle - she had a rev counter - I also
tried restricting the car’s engine revs, initially to 2,500 rpm (thus
allowing around 50 in fourth gear), then to 2,750 rpm (allowing better
hill climbing ability and a higher cruising speed). This made things a lot easier, and allowed a
higher cruising speed.
It was in the Fiesta diesel, Geoffrey, that “e-max” proved its worth. Not only was an easier car to drive gently
thanks to the high torque output, all available at low revs, but getting 65 mpg
was rather encouraging. By using my
“e-max” technique when times were hard, I was able to consistently eke out more
than 60 mpg, even in the middle of winter. Remember that this is changing up
through the gears at no higher than 10, 20, 30 and 40 mph, with a maximum speed
of 60 in fifth gear. “E-max” driving in
Geoffrey didn’t hold anybody up, and was much easier to do compared with the
Fiesta.
Melissa, the
Cinquecento, had low gearing, which made it easy to use the “e-max” technique,
and when times were hard, I could squeeze more than 56 mpg from the Cinq. However, owing to the Cinquecento’s small
engine with a low output, the e-max technique wasn’t so easy to maintain on
some long drives, especially those with hills.
I was fortunate in that when times were hard, I drove a lot of my miles
in
By the time I was driving Lucy, the Mondeo, things were a little bit better on the money
side of things, but I did still go through some lean spots. The “e-max” technique was certainly possible
in the Mondeo, but it was much easier to stick to a certain rev
limit. In the Mondeo TD, you could stick
to 2,300 rpm and still cruise at 60 mph, and record economy figures of around
46 mpg through my “normal driving.” If
you wanted to drive quicker, 70 mph indicated was around 2,700 rpm. The only way I could squeeze more than around
46 mpg was on a long run with most of the mileage on either a motorway or dual
carriageway (whereas with my previous cars, you could also return the best
economy figures on main roads). It
was only on such long haul drives that we would return more, managing to break
52 mpg on one summer run.
The Mondeo’s “turbo spot” was just
above 80 mph indicated and given my dislike of speeding, I very rarely cruised
at this speed.
Kermy benefits from cleaner
aerodynamics compared to Danielle
and taller gearing but on the other hand, he was heavier and has significantly
more emissions control gubbins than the earlier Ford engine. Fuel consumption has proved comparable if
volatile with a low of 26 and a high of just over 49 mpg. The average crept up over 41 mpg, but look here for an up-to-date figure.
For technique, the Endura-E has lots of low down torque
and using this, rather than chasing the power at higher engine speeds, produces
better results. Keeping the engine speed
below 3,000 rpm (this corresponds to approximately 65 mph indicated in top
gear) or better yet, 2,500 rpm (fifty five in top) produces the best
results. What really hurts the Ka’s fuel
consumption is urban driving. During cold running, Ford deliberately run
the fuel:air mixture especially rich and when using the air conditioning compressor, consumption is increased by
two thirds at a warm idle.
For an article on the Endura-E’s
efficiency and fuel consumption, see here.
The Accord’s fuel consumption was
advantaged by my motorway commute, 36 to the gallon from something reputed to
return 33·5 is good going. With the
Accord returning decent consumption figures is easy. The golden rule is, “if you can hear the
engine, it’s thirsty.” This means
sticking to under 3,500 rpm on the motorway, which is also inside the speed
limit. Using the higher lift camshaft
profile in the lower gears burnt a disproportionate amount of fuel, but, it did
sound good.
The Saab
9-3 is easy to drive along using the eMax technique and 2,500 rpm providing
you are in no hurry, since 2,000 rpm in top gear is 58 miles per hour indicated,
or 56 GPS-reported.
HGV Pace is simply that: you adopt
the same speed as a heavy goods vehicle, usually
reserved for the motorway. There are a
few HGVs that drive along at forty on a normal single carriageway road, but
most seen to aim for their limiter speed!
There are two advantages of
maintaining the HGV pace. One is that
when on a busy multilane road, you avoid the constant acceleration and
deceleration you'll find in the outside lanes.
You amble along at something like three seconds plus of road distance
from a heavy goods vehicle, which will be something like fifty five miles per
hour.
The other advantage is in fuel
efficiency, where the direct benefit is that you're cruising at a slower speed
and an indirect advantage is that you are no longer changing speed so much (changing speed wastes fuel).
It's not all plain sailing,
though. The disadvantage of heavy goods
vehicle pace is that if you usually cruise at seventy it'll can take you a fair
bit longer to arrive at your destination.
The other disadvantage is that many heavy goods vehicles cannot maintain
a constant speed through hilly regions, they decelerate on the way up and
accelerate on the way down. And the
final problem is that of boredom...
As for me, I'll happily adopt the
same speed as heavy goods vehicles during the rush hour and on my usual
commuter routes. There is an average speed penalty, typically my 28 mile
motorway commute takes 45 minutes at HGV pace but 38 minutes at the speed
limit.
However, the reward is in reducing
fuel consumption, which for Kermy, Hoshi and Chef has been around fifteen
percent.