What Affects Fuel Economy?

Why your real average differs from the brochure

Manufacturer fuel economy figures are measured in controlled laboratory conditions. Real-world driving involves traffic, hills, passengers, air conditioning, and varying speeds — all of which change your actual fuel consumption. Understanding these factors helps you interpret your numbers and make simple changes that add up.

Driving style

How you drive has the single largest impact on fuel economy that is within your control.

• Smooth acceleration uses less fuel than hard, rapid acceleration.
• Anticipating traffic and coasting to stops avoids wasted braking energy.
• Steady cruising at moderate speeds is more efficient than frequent speed changes.
• Excessive idling burns fuel without moving — turn off the engine for stops longer than a minute where safe.

Speed

Fuel economy typically peaks at moderate highway speeds (around 70–90 km/h for most cars). Above that, aerodynamic drag rises sharply. Driving at 120 km/h can use 20–30% more fuel than driving at 90 km/h over the same distance.

Traffic conditions

Stop-and-go city driving is significantly less efficient than steady highway cruising. Each acceleration from a stop requires substantial fuel, and braking wastes the energy spent getting up to speed. This is why your city average is almost always lower than your highway average.

Tyre pressure

Underinflated tyres increase rolling resistance, forcing the engine to work harder. A tyre that is 20% below the recommended pressure can reduce fuel economy by several percent and also causes uneven, faster tyre wear. Check your tyre pressure monthly — see our Tyre Pressure Guide for details.

Air conditioning

Air conditioning draws power from the engine, which increases fuel consumption. At low city speeds, AC can add 5–20% to fuel use. At highway speeds, the drag from open windows can offset the AC cost, so using AC on the highway is often more efficient than driving with windows down.

Vehicle weight and load

Extra weight means the engine must work harder to accelerate. Every 50 kg of additional weight can reduce fuel economy by roughly 1–2%. Remove unnecessary items from your boot, and avoid carrying roof loads when you do not need them — roof boxes also create significant aerodynamic drag.

Engine and mechanical condition

• Dirty air filters restrict airflow and can reduce efficiency, especially in older vehicles.
• Worn spark plugs cause incomplete combustion.
• Old or incorrect engine oil increases internal friction.
• Faulty oxygen sensors can cause the engine to run rich (use more fuel than needed).
• Dragging brakes create constant resistance.

Regular servicing keeps these components in good condition and protects your fuel economy.

Road and weather conditions

• Hills and inclines require more fuel on the way up; you recover some on the way down.
• Strong headwinds increase aerodynamic drag.
• Rain and wet roads increase rolling resistance slightly.
• Cold weather increases fuel use because engines take longer to reach efficient operating temperature and cold air is denser.

Short trips

Short trips (under 5 km) are particularly inefficient because the engine never reaches its optimal operating temperature. The engine runs rich (using more fuel) when cold, and the car spends a large proportion of the trip warming up rather than running efficiently. Combining errands into a single trip is one of the simplest ways to improve real-world economy.

Fuel quality

Using the fuel grade recommended by your manufacturer is important. Higher-octane fuel does not improve economy in cars designed for regular fuel — it simply costs more. Using lower-octane fuel than recommended, however, can reduce efficiency and potentially cause engine knock.

How to use this information

You do not need to change everything at once. The most effective, no-cost improvements are:

1. Check and correct your tyre pressure.
2. Smooth out your acceleration and braking.
3. Combine short trips where possible.
4. Remove unnecessary weight from the vehicle.
5. Keep up with regular servicing.

Track your average over several fill-ups before and after making changes, so you can see the real effect.

Related guides

• How to Calculate Your Car's Fuel Average
• Basic Car Maintenance Checklist