Quick refresher: L/100 km means how many litres your car uses to travel 100 km. Lower is better. If you prefer U.S. mpg for comparison, convert with mpg (US) ≈ 235.21 ÷ L/100 km. For imperial mpg, it’s ≈ 282.48 ÷ L/100 km. Example: 8.0 L/100 km is roughly 29.4 mpg (US).

Here’s the rub: a mate of mine in Wellington bought a compact rated at 6.0 L/100 km combined. Commuting up Ngauranga Gorge with a roof rack and a bit of spirited driving, he averaged 7.9. He didn’t buy a bad car. He just used a good number the wrong way.

What if you’re comparing the wrong number altogether?

Most shoppers treat fuel economy like a scoreboard: pick the lowest L/100 km label and you win. The better way is to treat it like a weather forecast. The number tells you what should happen under controlled conditions, not what will happen on your routes, with your habits, in your climate.

Think of the official “city” and “highway” ratings as treadmill tests. Agencies run standard drive cycles on a dynamometer: stop‑start urban patterns for city, steady cruising for highway, then blend them into a combined number (commonly a 55% city, 45% highway weighting using a harmonic average). Modern methods layer on extra cycles to simulate aggressive driving, air‑conditioning, and cold starts. It’s methodical and fair-just not your life.

Traditional buyer logic fails because it ignores two mismatches:

• Your duty cycle vs the lab cycle. Hills, speeds, loads, temps, and trip length matter-a lot.
• Your behaviour vs the test driver. Speed, acceleration, idling, and A/C use swing results more than most people think.

Better questions to ask yourself:

• Does the “city/highway” split match how you actually drive each week?
• Which test protocol is behind this label (EPA, WLTP, NEDC, JC08), and how does that compare across markets?
• What variables can I control that move the needle more than a 0.3 L/100 km spec-sheet difference?

How big is the gap between the label and your driveway?

Here’s the evidence behind that perspective shift, distilled for real NZ/AU use:

• Test cycles are controlled. Real roads aren’t. Official tests fix temperature, load, and speed profiles. In the real world, coastal headwinds on the Princes Highway, windy Tasman crossings, or the Kaimai Ranges change the game.
• The “real‑world gap” is well documented. Independent researchers have shown that on‑road fuel use is often worse than the lab number, which triggered new procedures like the EPA’s five‑cycle adjustments and Europe’s WLTP to better reflect everyday driving.
• Driver behaviour is the biggest wild card:
  • Aggressive driving (hard accel/braking) can worsen economy by roughly 10-40% in the city and 15-30% on highways.
  • Speeding hurts. Fuel economy drops quickly above about 80 km/h; slowing by 8-16 km/h can yield around 7-14% improvement.
  • Air‑con load is real. In hot conditions or short trips, A/C can push consumption up by more than 20% in conventional vehicles.
  • Weight and drag add up. Every 45 kg can hit economy by about 1%. Roof boxes can cost several to 10-25% at motorway speeds.
  • Under‑inflated tyres typically waste a few percent; correct pressures can recover up to ~3%.

What does a wrong pick actually feel like day to day?

Picture this. You choose an SUV because the brochure says 7.8 L/100 km combined. Summer arrives, you fit a roof pod for a family run from Christchurch to Wanaka. There’s a nor’wester, bikes on the back, and the boot full. The trip computer reads 10.4. You start rationing café stops to “save fuel” and turn the A/C down even though the kids are flushed. That faint feeling of “we bought the wrong thing” creeps in.

Flip the script. Same budget, but you picked a model with slightly better highway gearing and lower roof rails. You left the pod at home and packed soft bags instead. You set cruise at 100 km/h. Now you’re seeing 8.3. You arrive calmer, spend less, and feel quietly chuffed every time you pass a servo.

This isn’t about perfection. It’s about aligning your car, your driving, and your conditions so the number works for you-not against you.

How do you choose smarter? Use the DRIVE framework.

• D Duty cycle: Match ratings to your real split. Mostly city? Prioritise the city L/100 km and cold‑start behaviour. Mostly open road? Look hard at highway numbers and aero/gear ratios.
• R Ratings source: Know which test produced the figure. EPA five‑cycle, WLTP, older NEDC or Japan’s JC08/WLTC can differ substantially. Cross‑market comparisons aren’t apples-to-apples.
• I Influences: Identify what you control-speed, acceleration, roof racks, weight, tyre pressure, A/C use. These swing results more than tiny spec-sheet gaps.
• V Verify: Read owner forums and independent road tests in similar climates (Perth heat vs Dunedin cold is a different story). Check long‑term averages, not a one‑off hero run.
• E Expenses: Translate L/100 km into dollars per km for your local fuel price. That’s where smart trade‑offs become obvious.

A simple checklist to take to the dealership:

• What are the city, highway, and combined L/100 km values, and which test protocol are they from?
• How does this model perform in independent, on‑road tests?
• What eco drive modes, gearing, and tyre options affect real‑world economy?
• What’s my cost per 100 km at today’s $/L?

What exact steps should you take next (and what trade-offs)?

1. Start with the numbers you can compare. Note the city, highway, and combined L/100 km from the label and the test type behind them. If you mostly do short urban trips, weight the city number more heavily. For long commutes or rural drives, lean on the highway figure.
2. Convert to costs you feel. Fuel cost per 100 km = (L/100 km) × (price per litre). Example: At $2.50/L, a car rated 8.0 L/100 km costs about $20 per 100 km (20 cents per km). Improve by 10% to 7.2 L/100 km and it’s $18 per 100 km. That’s $200 saved over 10,000 km. If you drive 20,000 km a year, you’re looking at $400.
3. Consider the “free” gains before you pay for hardware. Driving habits and setup:
   • Smooth acceleration and anticipation: up to 10-40% better in city driving if you’ve been heavy‑footed.
   • Keep highway speeds near limits: 7-14% improvement by easing off 8-16 km/h.
   • Use cruise control on flat highways; avoid it in hilly terrain.
   • Minimise idling; modern engines use very little to restart compared to idling for minutes.
   • Remove roof racks/boxes when not needed and clear heavy clutter.
   • Combine errands to cut cold starts.
   Maintenance:
   • Check tyre pressures weekly or before long trips; aim for the manufacturer’s cold pressure.
   • Keep up with oil changes and engine checks. Faulty oxygen sensors or dragging brakes can quietly burn cash.
   • Use the recommended oil viscosity and fuel grade; the wrong oil can cost 1-2% in economy.
   • Consider low‑rolling‑resistance tyres if they suit your driving and safety needs.
4. Use in‑car tech to keep yourself honest. Watch instantaneous and average L/100 km on the trip computer. Reset per tank and per trip to learn what helps. Try Eco or efficiency modes; in many cars they tame throttle mapping and shift points. In a manual, upshift early without lugging. In an auto, avoid sport modes for daily driving.
5. Shop with your routes in mind, not the brochure route. If you test-drive, include a stretch that mimics your commute: a cold start, a hill, a bit of motorway. Note the indicated L/100 km and your speed. Ask the salesperson which tyres are fitted; high‑grip options can cost a little fuel. Get the safety you need, but know the trade-off.
6. Understand the controversies so you’re not blindsided. Different protocols produce different numbers. WLTP often shows higher consumption than older NEDC for the same car; EPA’s five‑cycle is tougher than single‑cycle tests. Cross‑shop using the same protocol where possible. Real‑world gaps vary. Audits and NGO studies have pushed regulators to tighten procedures, but you should still expect some spread between label and life. Manufacturer vs observed data can differ. That’s why independent tests and owner reports are gold.
7. Plan for your household, not the “average” driver. Compact/urban drivers: Cold starts dominate. Prioritise strong city ratings and gentle driving; consider features like stop‑start and efficient A/C. Highway commuters: Focus on aerodynamics, gearing, and a calm cruise speed. Remove roof add‑ons when you’re not using them. Towing or loads: Expect higher L/100 km. Plan routes and speeds accordingly, keep tyres inflated for the load, and budget the extra fuel in your costs.

Your goal isn’t to win a spec-sheet contest. It’s to pick a car and a set of habits that deliver the economy you’ll actually see on SH1 in a crosswind, up the Toowoomba Range on a hot arvo, or during a winter school run in Christchurch. Use DRIVE, sanity‑check the label against your life, and turn that L/100 km from a hopeful promise into a predictable outcome.