It may seem odd for a fuel company to be encouraging people to buy less fuel.
Even Z Energy, which is sponsoring "eco-driving" courses around the country to teach motorists how to trim their fuel bills, admits the exercise could appear "counterintuitive".
"But we're looking further ahead than the next tank of gas we sell," spokeswoman Christine Langdon says of the courses run by racing car engineer-turned-fuel conservationist Mark Whittaker and his Auckland-based company, ecodriver.
"Our business can only be sustainable in a sustainable world. Even though it might mean customers spend a little less on fuel each time they visit, or need to fill up a little less often, we reckon it's still better overall for our bottom line."
The initiative has been welcomed by the Automobile Association and the Sustainable Business Network, which say although the ultimate way to save fuel is not to drive at all, that's not realistic for most people given a lack of suitable transport alternatives.
Mr Whittaker has worked more than 25 years in the motor industry and, as well as winning fuel-efficiency awards with racing legend Chris Amon in Energywise rallies, has driven a Mini Cooper from Cape Reinga to Bluff on $77.12 worth of diesel - with enough left from his second fill-up, in Wellington, to double back to Dunedin.
That worked out at just 3.5 litres for each 100km driven.
"People ask how motor racing and fuel economy go together," Mr Whittaker says. "But there's a lot of allied stuff in it - on a racetrack, people figure out how to conserve momentum, or the energy of the car, so they can build from that base."
The driver has the biggest impact. You can drive a Prius [hybrid] and get terrible fuel economy if you want to.
Drivers don't have to sacrifice much time to reduce fuel costs, although optimum speed for most cars is 60km/h, after which fuel consumption increases disproportionately.
His average for the Mini run - during which he admits to a speeding ticket in Oamaru a la Goodbye Pork Pie - was 78km/h to 79km/h.
The main trick is to keep the speed as constant and smooth as possible, realising each squeeze of the accelerator squirts a large amount of energy into the engine, compared with hardly any needed in cruise mode.
That means being sparing with the brakes, and looking much further down the road than most drivers do, to judge exactly when to lift off from the accelerator and go into cruise mode.
But Mr Whittaker says it's wrong to think cruising in neutral in fuel-injected cars - which now account for most of the country's fleet - will save more energy than staying in gear while doing so.
In fact, more fuel is needed to keep the engine ticking over in neutral
Although the 20 places in the Z-sponsored courses this week and next have been filled, the company is looking forward to participants taking part in follow-up sessions in about six weeks, to assess improvements, before a light-hearted playoff day at a Wellington racetrack.
Mr Whittaker says most of his clients are vehicle fleet operators looking for cost savings, but also with an eye on the environment. They include an insurance company sharing cost savings with staff and a tyre firm running a "name and shame" board for those who haven't taken his lessons seriously enough.
Although the industry spends billions a year on research and development in pursuit of the most fuel-efficient cars, not much attention has been given to teaching drivers.
"The driver has the biggest impact," Mr Whittaker says. "You can drive a Prius [hybrid] and get terrible fuel economy if you want to."
Mark Whittaker's top 10 fuel-saving tips
1. Look well ahead - to assess when to lift your foot off the accelerator to coast to a halt rather than braking.
2. Accelerate evenly - while picking an optimal target speed for terrain and traffic conditions, and sticking to it.
3. Keep good following distances - at least two or three seconds, so as not to simply react to what the driver in front is doing.
4. Drive as smoothly as you can - by imagining you are balancing a bowl of eggs on your bonnet.
5. Check tyre pressures - preferably every time you fill up, but at least once a month, as low pressure can cost 5 per cent more fuel.
6. Use the highest gear possible - without causing your car to judder up hills. To change up in an automatic car, lift your foot off the accelerator, then "feather" it back on the pedal to maintain speed.
7. Watch your speed - the optimum speed of most cars for fuel efficiency is about 60km/h, after which the energy needed to push through the air goes up disproportionately.
8. Turn off air-conditioning when not needed, or face an 8 per cent to 12 per cent fuel penalty.
9. Lighten loads - every 50kg added to a car costs 1 per cent to 2 per cent more fuel.
10. Reduce drag - a roof rack will disturb air flow over your car. And at speeds over 50km/h, driving with windows down will use more fuel than keeping aircon on.
• Click here for more tips on saving fuel costs.