August Fuel Costs Rose 28.8% on the Same Fleet Mix

In August, Geostat's detailed fuel items were +29.6% for gasoline, +48.7% for diesel fuel, and +0.4% for gas fuel year on year. A simple headline can therefore be dominated by diesel even though diesel represented only 4.3% of the fixed advertised-fleet mix used here.

The Fleet Fuel Cost Index applies the same three official item movements to the July issue's composition and consumption assumptions. It rose 28.8% year on year and 0.3% from July to August.

• Fleet Fuel Cost Index, year on year — August change: +28.8%
• Fleet Fuel Cost Index, month on month — August change: +0.3%
• Diesel fuel, year on year — August change: +48.7%
• Gasoline, year on year — August change: +29.6%
• Gas fuel, year on year — August change: +0.4%

The difference between +28.8% and +48.7% is the effect of applying each fuel's price movement to the fuel mix and assumed litres burned by the cars in the basket. Diesel's item price rose faster; the fleet index reflects all three fuel items.

Applying the August prices to the unchanged age-band weights gives a narrow annual index range: +28.4% to +29.4%, a spread of 1.0 percentage point. The fuel volume implied by the same assumptions is much wider.

• 0–3 years — Fuel coverage: 65.4%; Assumed litres per 100 km: 7.40; Index, year on year: +28.4%; Index, month on month: +0.4%
• 4–7 years — Fuel coverage: 71.7%; Assumed litres per 100 km: 8.07; Index, year on year: +29.4%; Index, month on month: +0.4%
• 8–12 years — Fuel coverage: 75.5%; Assumed litres per 100 km: 8.66; Index, year on year: +28.6%; Index, month on month: +0.3%
• 13 years and older — Fuel coverage: 84.1%; Assumed litres per 100 km: 8.98; Index, year on year: +28.4%; Index, month on month: +0.2%

The oldest band carries 21.3% more assumed fuel volume per 100 km than the newest band (8.9795 versus 7.4032 litres). The volume difference sits alongside annual fuel-price increases of roughly 28.4% for both groups.

The mix explains why the price index stays close. The newest band is 44.6% electrified (hybrid, plug-in hybrid, or electric) and only 0.7% diesel. In the oldest band, electrified cars are 13.7%, diesel is 11.1%, and LPG is 7.3%. Diesel pushes the oldest band's price index upward; LPG, with an annual move of only +0.4%, offsets part of that pressure.

The same fixed vintage also permits a price-band cut. The annual spread is wider than the age spread: +27.9% in the $20k-and-above band against +29.3% in the $5k–10k band.

• $2k–5k — Fuel coverage: 90.8%; Assumed litres per 100 km: 8.61; Index, year on year: +29.0%; Index, month on month: +0.3%
• $5k–10k — Fuel coverage: 82.3%; Assumed litres per 100 km: 8.40; Index, year on year: +29.3%; Index, month on month: +0.3%
• $10k–20k — Fuel coverage: 67.1%; Assumed litres per 100 km: 8.26; Index, year on year: +29.2%; Index, month on month: +0.4%
• $20k and above — Fuel coverage: 68.6%; Assumed litres per 100 km: 7.92; Index, year on year: +27.9%; Index, month on month: +0.3%

The highest-priced band has the lowest assumed volume, consistent with its greater hybrid and electric share and lower LPG-weighted burn. The difference is modest in percentage-price terms, but meaningful in the implied litres per 100 km.

For a household comparing cars, the index and consumption columns serve different purposes. The index describes the annual fuel-price movement; the consumption assumption indicates the amount of fuel needed for the same distance.

In this basket, the newest age band implies 7.40 litres per 100 km, against 8.98 litres for the oldest. The $20k-and-above price band implies 7.92 litres, compared with 8.40 litres in the $5k–10k band. Actual bills depend on the car, driving conditions, distance and fuel price paid.

Fuel type is stated unevenly across the advertised offer, so the tables show coverage beside each segment.