How to calculate and reduce your vehicle's fuel costs

L/100km vs MPG: the maths

L/100km measures fuel used over a fixed distance — lower is better. MPG measures distance covered per unit of fuel — higher is better. They describe the same thing from opposite ends, which is why you cannot average MPG figures meaningfully but you can average L/100km.

• L/100km = litres ÷ kilometres × 100
• US MPG → L/100km = 235.215 ÷ MPG
• UK MPG → L/100km = 282.481 ÷ MPG
• Cost per km = L/100km ÷ 100 × fuel price per litre

Measure your real consumption in five steps

1. 1. Fill the tank completely and note the odometer

   Brim the tank, then write down the odometer reading. This is your starting point — accuracy here matters more than anything else.

2. 2. Drive normally until the next fill-up

   Cover at least one full tank of your usual mix of driving. A single short trip will not give a representative figure.

3. 3. Brim the tank again and record litres and distance

   The litres on the pump is exactly what the previous distance consumed. Subtract the odometer readings to get the distance.

4. 4. Calculate L/100km

   Litres ÷ kilometres × 100. For example, 52 litres over 610 km is 52 ÷ 610 × 100 = 8.52 L/100km.

5. 5. Turn consumption into a cost per kilometre

   L/100km ÷ 100 × fuel price. At 8.52 L/100km and 24.00 per litre that is 2.04 per kilometre, or roughly 2,045 for every 1,000 km driven.

Nine ways to cut your fuel bill

• Check tyre pressure monthly. Under-inflation of 20% can add roughly 3% to consumption and destroys tyre life at the same time.
• Drop your cruising speed. Aerodynamic drag rises with the square of speed; 120 km/h instead of 130 km/h typically saves 8–12%.
• Remove roof racks and boxes when not in use — an empty roof box can cost more than 10% on the highway.
• Lose dead weight. Every extra 50 kg costs roughly 1–2% in town driving.
• Combine short trips. A cold engine can use double its normal fuel for the first few kilometres.
• Anticipate instead of braking. Every braking event throws away fuel you already paid to convert into speed.
• Service on schedule. Clogged air filters, tired spark plugs and old oil all quietly add percentage points.
• Use the recommended oil grade. A thicker oil than specified increases internal friction and consumption.
• Track every fill-up. A rising trend is usually the first sign of a mechanical problem — and the cheapest time to fix it.

Frequently asked questions

How do I calculate fuel cost per kilometre?

Divide your consumption (L/100km) by 100, then multiply by the fuel price per litre. At 8.5 L/100km and a price of 24.00 per litre, that is 0.085 × 24.00 = 2.04 per kilometre.

How do I convert MPG to L/100km?

Divide 235.215 by the US MPG figure. 30 US MPG is 235.215 ÷ 30 = 7.84 L/100km. For imperial (UK) MPG, divide 282.481 by the MPG figure instead.

What is a good fuel consumption figure?

It depends on the vehicle class. A small hatchback typically sits near 5–7 L/100km, a mid-size SUV near 8–10 L/100km, and a loaded double-cab bakkie or pickup near 9–12 L/100km. Compare against your own history rather than a brochure number.

Why is my real consumption worse than the manufacturer claim?

Official figures come from a controlled test cycle. Short trips, cold starts, roof racks, low tyre pressure, heavy loads and highway speeds above 110 km/h all push real-world consumption 10–30% higher.