How to Track Your Car's Fuel Efficiency

Most drivers have no idea how much fuel their car actually uses per kilometre. They fill up when the gauge is low, pay what the pump says, and move on. This means small inefficiencies — a slightly underinflated tyre, a dirty air filter, or a change in driving route — go unnoticed for months, costing hundreds of dollars a year.

Tracking fuel efficiency takes less than 60 seconds per fill-up and gives you a clear, data-driven picture of what your car is actually doing.

The Two Key Metrics

Fuel efficiency is expressed in one of two ways depending on where you live:

• L/100km (litres per 100 kilometres) — used in Australia, Europe, South Africa, and most of Asia. Lower is better. A reading of 6.0 L/100km means you use 6 litres to travel 100 km.
• km/L (kilometres per litre) — used in Japan, India, and parts of Southeast Asia. Higher is better. A reading of 16 km/L means you travel 16 km on each litre of fuel.

Both metrics describe the same thing from opposite directions. To convert: L/100km = 100 ÷ km/L. So 6.25 L/100km = 16 km/L.

How to Calculate L/100km Manually

You need two data points from consecutive fill-ups:

• The odometer reading at fill-up A (start)
• The odometer reading at fill-up B (next fill-up)
• The number of litres pumped at fill-up B

The formula: L/100km = (litres pumped ÷ distance driven) × 100

Example: You fill up at 45,200 km. Next fill-up is at 45,540 km and you pump 22.4 litres. Distance = 340 km. L/100km = (22.4 ÷ 340) × 100 = 6.59 L/100km.

What Affects Fuel Efficiency

• Tyre pressure — underinflated tyres increase rolling resistance. Check pressure monthly. A tyre that is 10 PSI low can increase fuel consumption by 2–3%.
• Driving style — aggressive acceleration and hard braking are the single biggest variables in fuel use. Smooth, anticipatory driving can improve efficiency by 15–30%.
• Air conditioning — running the AC increases fuel consumption by 5–15% depending on vehicle size and ambient temperature.
• Vehicle load — carrying an extra 50 kg increases fuel consumption by roughly 1–2%. Remove roof racks and heavy items from the boot when not needed.
• Trip type — short trips under 5 km are disproportionately inefficient because the engine never reaches optimal operating temperature.
• Engine condition — a sudden drop in efficiency often signals a mechanical issue: dirty fuel injectors, a failing oxygen sensor, or a clogged air filter.

How to Log Fill-Ups Consistently

• Always fill to full — partial fills make calculations unreliable
• Record the odometer reading before you start pumping, not after driving away
• Log every fill-up, not just memorable ones — the average over 10+ fills is far more useful than individual readings
• Note if the fill was partial (travelling, different station) so you can flag it in your records

When to Investigate

A single bad fill-up reading can be explained by driving conditions. But if your rolling average worsens by more than 10% over three or more consecutive fill-ups, investigate:

• Check and adjust tyre pressure
• Check and replace the air filter if overdue
• Check for dashboard warning lights
• Book a service if no obvious cause is found

Frequently Asked Questions