Prime
The gentle art of fuel economy
Motorists stuck in traffic jam on the Northern by-pass. Open road cruising at a steady speed is more fuel efficient than the stop-start of heavy traffic. PHOTO/MICHAEL KAKUMIRIZI.
What you need to know:
With fuel prices remaining high, here are some factors to put in mind while driving in order to save fuel.
By Gavin Bennet
Efficient driving involves you accelerating more gently, getting into a higher gear a bit sooner, moderating your cruising speed and easing off earlier before you start to brake. Everything smoothly and gently, as if you have a full bowl of water on the dashboard and do not want to spill it.
This will not only optimise your fuel economy, but will save parts such as the brakes, tyres, engine, transmission and passengers from damage. There are five main factors which determine how many kilometres your car will go on a litre of fuel.
Vehicle design
The first is the design of the vehicle itself, especially the size and design of the engine; the size, weight and aerodynamic shape of the body; the power-weight ratio, and also its gearing and fuel induction system (carburetor and fuel injector, among others). This is each make and model of vehicle’s in-built design efficiency (or otherwise). It sets absolute top and bottom limits on fuel consumption, but the potential range is huge (on an average 1600cc car anywhere from six kilometres per litre to 16 kilometres per litre.
Service condition
This includes the state of wear in the engine and the whole transmission train, the setting and lubrication of all moving parts, the tuning of fuel/air mixture, the timing, and the condition of the ignition (plugs and points), wheel alignment and tyre pressure, among others.
Again, these do not determine what your final fuel consumption will be, but they will either optimise or diminish the car’s economy potential quite significantly, depending on whether the car is in perfect running order or seriously worn or defective. Good condition will not only optimise economy, it will also deliver good performance when you want or need it.
Road condition
A hard, smooth surface absorbs much less energy than a soft or rough surface; straight and flat is more economical than twisty or hilly; open road cruising at a steady speed is more efficient than the stop-start of heavy traffic, and so on. Even the temperature and weather can have some effect, cool and damp being best.
Load
A fourth factor is the load the vehicle is carrying. It is an immutable fact of physics that the heavier the weight that has to be moved, the more energy is required to move it. So, every kilo of weight you add to your car will increase the amount of fuel it consumes.
The greater the increase in weight, the greater the increase in fuel consumption.
You can exercise a certain amount of control over these factors (especially service condition) but in practice for most motorists most of the time, theory meets a reality check; this is the vehicle you have, this is the condition it is in, this is the road it is travelling, and this is the load it is carrying.
Driving
Now; what can still be done to make fuel consumption either better or worse? Enter factor number five; the way you drive.
Even if all other factors (design, condition, road and load) are equal, driving technique alone can make a huge difference to fuel consumption and to general wear-and-tear.
At one extreme is the driving style of rallying, characterised by phrases such as “foot flat” and “max revs”.
In translation, the driver’s right foot is always pressed hard to the floorboards either on the accelerator or the brake; the rev counter needle is always near the red line, in the lowest gear possible for maximum power, steering movements and gear changes are frequent and violent. All done on split-second and last-second reflexes.
This technique in the quest for the highest possible speeds at all times not only maximises fuel consumption, it also imposes maximum stress and wear on all working parts of the engine and transmission and suspension and brakes, and if the car is powerful and there are plenty of corners, it can tear a brand new set of tyres to shreds in less than an hour.
Driven in this way, the average 1600cc family car would guzzle fuel at the rate of about six kilometres per litre and, incidentally, would need a new set of tyres and brakes every 1,000 kilometres, and be a worn-out wreck before it reached 10,000 kilometres.
This is relevant to the ordinary motorist, because if this is the way to maximise fuel consumption and wear, then precisely the opposite technique is what will minimise fuel consumption and wear.
What to avoid
Never foot flat. Never max rev. Never drive violently. Go for mostly gentle and gradual with the lightest possible touch, whether you are accelerating or cornering or braking.
Mostly in the highest possible gear without labouring the engine. Mostly at moderate speeds. And always anticipating the road and traffic ahead, so nothing has to be done at the last second.
A car kept in good condition and driven skillfully in this way will have a fuel economy more than 100 percent better than the “rallying” extreme, and probably 20 percent better than the average achieved in “ordinary” motoring.
To save fuel:
Gradual acceleration and easing off well before a slowing or stopping point, so you hardly have to brake at all and then only softy. You might sometimes have to (and should) compromise these principles in busy highway traffic, especially when overtaking.
Do not use up the rare moments when the road ahead is all-clear of oncoming traffic by dawdling. Change down a gear and pass under full power.
If your car has a rev counter, you should generally motor between 2,000 and 3,000 rpm except in emergencies or other exceptional circumstances (such as overtaking). Every gear change should take place at or before 3,000 rpm.
In fifth gear, that will be a cruising speed of about 100 kilometres per hour for most cars.
If you do not have a rev counter, use that 100 kilometres per hour in fifth gear as your benchmark and remember the engine note. Routinely, do not work the engine harder than that in any gear.
In all respects, think half. Half the vehicle’s maxium revs, acceleration or speed. Half its payload.
It is then working well within its design capacity and thus never strained. That means minimum wear and minimum fuel consumption.
Your lifestyle (or travel itinerary) may not readily conform to this gentle driving pattern.
So be it.
But then the higher cost of your motoring will be your own choice. It is possible to ensure lower fuel consumption, lower maintenance bills, less wear and tear. Optimum economy does not lie in short-cuts and neglect. It lies in care and doing things properly, both behind the wheel and in the workshops.
Remember to...
Do not neglect routine service items:
Lubrication, tyre pressures, plugs and points, filters. Over regular service intervals, keeping these items in top shape will save you more in petrol than it will cost you in maintenance. Delaying or neglecting service is a false economy.
