Ask the mechanic: Why has our vehicle fleet fuel consumption increased?

Hello Paul, I run a fleet of Toyota Land Cruisers. Two months ago we hired new drivers and since then our fleet management system shows that the fuel consumption of these vehicles has gone up. What could be the cause? William

Hello William, excessive fuel consumption is when your vehicle or fleet of vehicles seem to consume up to 30 percent more fuel over a period.

Not only does monitoring fuel consumption show the fleet running costs but also helps fleet managers understand vehicle health as well as the driving habits of their drivers.

The common causes of increased vehicle fuel consumption are usually grouped under vehicle maintenance or driving style or habit. 

Vehicle maintenance usually considers lubrication system or engine oil, ignition system, emission system, tyre management and use of air conditioning. 

Lubrication or engine oil 

Old engine oil usually becomes sludgy or lumpy. This affects the oil viscosity or ability to flow and get to where it is needed in the engine for cooling, cleaning and improving performance.

As a result, the engine moving parts run hotter and heavier (meet more resistance). This will increase inefficient engine running and fuel consumption.

Choose the manufacturer recommended oil viscosity index to ensure quick flow. Reputable engine oils are designed with detergency and efficiency improver additives to improve fuel consumption by keeping the engine running clean and light.

Emission and air intake systems

Components such as the oxygen (O2) sensors help the engine management system regulate the amount of fuel delivered. These sensors wear out with time and lead to increased fuel consumption.  O2 sensor failure can cause a check engine light and can be detected using a computer diagnostic tool.

Air intake components such as the air cleaner is crucial for trapping dust and road soot as well as letting in clean air. When it is clogged, it restricts air flow and affects the air fuel ratio which causes fuel wastage during combustion.

Dirty air cleaners eventually let in dust and soot which damages engine management components such as the air flow metre and throttle potentiometer, which affect air fuel ratio and increase poor fuel economy.

Tyre management

Suitable well inflated tyres with recommended tread depth improve car handling as well as reduce rolling resistance. This is a crucial factor for good fuel economy. When tyres are under inflated or have worn out treads, the car uses more energy (fuel) to attain the required speeds.

Air conditioning (AC) is a convenient feature and good for comfort. However its an additional load to your engine. When you run the AC your fuel consumption increases. 

Driving habits or style also increases fuel consumption. Consider the driving styles of your fleet drivers. Unnecessary idling of the engine when you are going nowhere will increase your fuel consumption. Choosing the right gear is an important way to prevent fuel wastage. 

This can be done by listening to your engine and knowing when it needs lower or higher gears. When carrying heavy loads or driving uphill, select lower gears. When driving faster use the higher gears. Aggressive driving causes poor fuel economy. 

Driving too fast, quick acceleration and sudden stops all increase fuel consumption. Gentle acceleration blending with the traffic and safe distancing helps to avoid sudden stops as you use momentum to gain speed or gears to help slow the car.

The ignition system

Include spark plugs or fuel injectors whose working condition affects efficient burning of fuel. The spark plugs wear out with time and need to be inspected or replaced where necessary during service.

Fuel injectors are crucial for delivering the right amounts of fuel for combustion. Dirty petrol or diesel injectors will deliver unregulated and wasteful amounts of fuel. Fuel injectors can be kept clean by regularly maintaining the fuel filter.