In the first part of this series we looked at choosing the most economical
car to suit your needs. (See Savings on Fuel - Part One.)
In this, the second part, we’ll look at maintenance that will keep your
vehicle running at its most frugal.
Engine Oil and Gear
To wring maximum fuel economy from your car, you should use the thinnest
engine oil and gear lubricants recommended by the manufacturer.
As a guide, most modern engines require a multi-grade oil with a viscosity of
around SAE 10W/40. This describes an oil with a viscosity rating of 10 when cold
and 40 once warmed to normal operating temperature (the higher the number, the
thicker the oil). A common alternative for SAE 10W/40 oil is SAE 10W/30 – an oil
that gives identical lubrication characteristics when cold but stays relatively
thin once up to normal operating temperature. The SAE 10W/30 oil will reduce
engine losses and improve fuel economy without compromising engine
Eeach car has different engine oil requirements so you should always check
your manufacturer’s handbook.
Manual gearboxes and differentials require a dedicated gear lubricant ranging
from SAE 60 to 250. This is considerably thicker than typical engine oil. As a
guide, the early ‘90s Subaru Liberty can use either SAE 85 or 90 grade fluid for
its manual gearbox/front differential assembly. The thinner SAE 85 grade fluid
will deliver superior fuel economy without a major effect on lubrication but,
again, we recommend consulting the vehicle handbook for recommendations to suit
your driving environment.
When it comes to automatic transmissions, there is typically only one type of fluid
recommended by the manufacturer – and that’s the one you should use. Automatic
transmission fluid is extremely complex and should not be messed around with.
There are three engine-related filters in a car – the air filter, oil filter
and fuel filter.
Of this trio, the air filter can have the biggest effect on fuel consumption
- a dirty air filter restricts airflow into the engine, causing a noticeable loss
of power and increase in fuel consumption.
One solution is to replace the air filter at - or before - the recommended
interval specified in your vehicle handbook. An early ‘90s Subaru
Liberty requires an air filter
inspection every 25,000km/12 months and replacement every 50,000km/24 months. If
you drive in dusty conditions, we recommend more frequent inspection for air
Some people choose to install a generic aftermarket air filter. Many aftermarket air filter manufacturers claim superior
airflow to the factory part and, as a result, better power and economy. But a free-flow air filter may not provide engine protection equal to the
The fuel filter of your vehicle is another item that should be replaced at regular
service intervals. A clogged fuel filter can cause poor driveability and, as a
result, increased fuel consumption. You can help prevent a clogged fuel filter
by filling the car’s fuel tank and driving it to near empty rather than making
numerous small stops – this reduces the chance of foreign matter entering the
fuel tank through the filler neck.
The engine oil filter should be replaced at each oil change. However, of the three filters, the oil filter generally has the least effect on fuel
Your car’s engine should receive more than just routine fluid and filter
changes - engine tune is often overlooked.
In most fuel injected vehicles, the engine’s air/fuel ratio does not require
regular adjustment - the appropriate air-fuel ratios (ie mixtures) are
maintained by the engine management computer. However, if your car is returning
worse than expected fuel economy, it is worth checking two aspects of the fuel
system – fuel pressure and injectors.
For a small outlay, any mechanical workshop should be able to perform an on-car fuel pressure
test. If the fuel pressure is not within the manufacturer’s
specified range it’s important that you quickly repair the fuel system to
restore fuel economy.
If the fuel injectors are partially blocked (a problem often identified by
rough running) you should first try adding a bottle of injector cleaner
to your fuel. Injector cleaner is available at all petrol stations.
Should a bottle of injector cleaner fail to unblock the injectors, you’ll need
to remove the injectors and take them to a specialist workshop to be ultrasonic
The correct operation of all engine sensors is also essential.
Most fuel injected vehicles employ at least one oxygen sensor in the exhaust
system to monitor air/fuel ratio. The input from this sensor is used to
determine how much fuel needs to be injected into the engine for each cycle. If
the oxygen sensor is giving a false signal or fails, it will cause the engine to
run too rich or lean at light load. It’s also likely that a ‘Check Engine’
warning light will appear on the dashboard when the oxygen sensor - or any other
engine management sensor - fails. It’s important to replace faulty engine
sensors to maintain efficient engine operation.
In cars fitted with a replacement timing belt or chain, it’s a good idea to
check that the camshaft and ignition timing are set to factory specifications.
Incorrect cam and/or ignition timing can reduce fuel consumption and potentially
cause engine damage. Any workshop should be able to check timing for little
The ignition system is also are important from a fuel consumption point of
Spark plugs should be in good, clean condition and provide appropriate spark
energy to burn the air-fuel charge. A spark plug that is worn or has a too-wide
gap struggles to create a spark to ignite the charge - this means poor
combustion efficiency and poor fuel consumption. Use the manufacturer’s
recommended spark plugs with the appropriate gap and replace at the specified
Other engine tune factors that influence fuel economy include idle speed,
condition of the ignition system (coils, leads, distributor cap, rotor and other
parts specific to your car), engine compression, operation of crankcase
ventilation system and exhaust backpressure. Excessive exhaust back pressure is
commonly caused by a collapsed cat converter or a dislodged baffle inside a
One of the easiest steps to improve fuel economy is to ensure tyres pressures
are kept high.
For normal driving conditions, we suggest inflating your car’s tyres about 2 –
3 psi over the manufacturer’s recommendation. There should be no concerns with
over-inflation so long as you stay below the maximum inflation pressure of your
particular tyres (the maximum pressure is shown on the sidewall of the tyre).
Increasing tyre pressure reduces rolling resistance which aids fuel consumption.
Handling and steering response will also be improved but ride quality will be
The wheel alignment of your vehicle should also be within specifications
quoted by the vehicle manufacturer. Poorly adjusted wheel alignment causes
increased tyre drag and so increased fuel consumption. A wheel
alignment should be part of your vehicle’s regular service schedule.
More important considerations are wheel bearing and brake drag.
A deteriorating wheel bearing (which is often identified by a road speed
related rumble) reduces fuel economy by causing extra drag. Similarly, it’s
important that the car’s brakes are not dragging. Worn brake calipers and an
incorrectly adjusted handbrake are the common problems to watch out for. These
items should be checked as part of routine servicing.
Unnecessary Weight and Aerodynamic
Unnecessary weight causes poor fuel consumption.
If your car is packed with items such as golf clubs, a tool set, fold-up
chairs and an umbrella, you should be aware that you’re paying
extra for these items to travel with you. The heavier your car, the more energy
it needs to drive from point A to point B – and more energy means more fuel.
Take a good look through the boot, glovebox, centre console and beneath the
seats of your car – the more junk you can remove, the better.
Car aerodynamics also play a part in fuel consumption – especially when the
vehicle is used for a lot of high-speed driving.
During normal servicing it’s common for car undertrays to go missing –
they’re often seen as ‘unnecessary’. Undertrays should be reinstalled to
optimise performance of the cooling system and maintain under-car airflow. On
the other hand, add-on body accessories such as roof racks and spot lights
should be removed after use. These reduce a car’s aerodynamic efficiency and
increase fuel consumption.
In Part Three of this series we’ll take a look at driving techniques to
reduce fuel consumption...