Fuel Saving Myths That Are Costing You Money

The internet is full of tips for cutting your running costs at the bowser, and your mates aren't short of opinions on the matter either. The problem is that a surprising number of the most commonly shared pieces of advice are either completely wrong, massively overstated, or only true in conditions that don't apply to most everyday Australian drivers.

What's worse is that some of these myths aren't just useless , they're costing you money directly. Either you're spending on products or behaviours that do nothing, or the 'saving' strategy you're following is creating its own hidden cost somewhere else in your running expenses.

This guide runs through the most persistent fuel-saving myths circulating in Australia right now, explains why each one doesn't hold up under scrutiny, and , where relevant , tells you what actually does work instead. No gimmicks, no product pitches, just straight-talking analysis of what the evidence actually shows.

Myth 1: Premium Petrol Will Improve Your Economy in Any Car

This is one of the most expensive myths in everyday motoring, because acting on it costs real money on every single fill-up. The belief goes: premium petrol is better quality, cleaner, more refined , therefore it must be more efficient and will help any car perform better.

What Actually Happens

Octane rating , the 91, 95, or 98 number on the bowser , measures a petrol's resistance to knock (premature ignition). It says nothing about energy content, cleaning properties, or general quality. A 98 RON petrol has essentially the same energy per litre as 91 RON petrol. The octane number is about chemistry, not calories.

When you pour 95 or 98 into an engine designed and calibrated for 91, nothing special happens. The engine management system doesn't 'detect' the superior product and reward you with better economy or power. The engine simply runs the same way it always does, using the same amount of fuel, and you've paid 8–15 cents per litre more for the privilege.

When Premium Petrol Is Genuinely Worth It

If your vehicle specifies 95 or 98 as a minimum , check the fuel filler flap , then using the correct grade is not a luxury, it's what the engine was designed for. In this case, using the cheaper 91 actually causes the engine's knock sensor to retard ignition timing, reducing efficiency and sometimes making 91 the more expensive choice per kilometre despite being cheaper per litre.

The myth in numbers: a driver filling a 60-litre tank weekly, paying 12 cents per litre extra for unnecessary 95 RON, spends approximately $375 more per year for zero return. That's the myth, quantified.

Myth 2: Fuel Additives and Treatments Save Money

Walk into any Repco or Supercheap Auto and you'll find a shelf of products promising to restore engine performance, clean your injectors, boost octane, and improve fuel economy. They're marketed persuasively, priced between $15 and $50, and they have enthusiastic testimonials attached to them online.

What Independent Testing Consistently Shows

Independent testing , from consumer organisations, automotive engineering bodies, and the NRMA , consistently finds that fuel additive products sold in the Australian market deliver no measurable fuel economy improvement in well-maintained modern vehicles. The base-level detergent additives in quality branded petrol already keep injectors and combustion chambers clean under normal use.

Some products marketed as 'octane boosters' do marginally raise octane levels but typically not enough to meaningfully affect engine management in a real-world scenario , and the improvement in octane is not commensurate with the cost of the product.

The Exception Worth Knowing

If a specific fuel system issue , injector deposits on a high-mileage direct injection engine, for example , has been professionally diagnosed, a targeted professional-grade injector cleaner used through a specific protocol can provide a genuine improvement. This is different from pouring a $20 bottle of generic 'engine treatment' into a tank and expecting broad results.

The myth in numbers: a $30 additive promising 5% economy improvement on a $250/month fuel spend would need to save $12.50/month to break even. Independent testing doesn't support this outcome.

Myth 3: Warming Up the Engine Saves Fuel

There's a generation of Australian drivers who were taught to let the engine idle for several minutes before driving , particularly in cold weather , to 'warm it up properly' before putting it under load. The reasoning made sense for carburettor engines of decades past. For modern fuel-injected vehicles, it's the opposite of good practice.

Why It Actually Costs You

A modern fuel-injected engine warms up fastest by being driven gently, not by idling. During idling, combustion temperatures are lower and the warm-up period takes longer than it would under gentle driving load. Meanwhile, the engine is burning fuel with no forward progress , consuming petrol at roughly 0.5–1.0 litres per hour while delivering zero kilometres.

The extended idle also delays the catalytic converter reaching operating temperature, meaning emissions are higher during the warm-up phase than they need to be. And it delays the oxygen sensor coming online, so the engine runs open-loop (without the normal feedback correction) for longer.

What You Should Do Instead

Start the engine and drive immediately at a moderate pace , not hard acceleration, not flooring it , and give the engine 3–5 minutes to reach operating temperature through gentle use. This is faster, more efficient, and better for the engine than idling in the driveway.

The myth in numbers: 5 minutes of idling daily at 0.7L/hr uses approximately 21 litres per year , about $42 worth , without moving the car a single metre.

Myth 4: Turning Off the Engine at Every Stop Saves Significant Money

With the rise of factory stop-start systems in modern vehicles, there's a version of this that's legitimate. But the DIY version , manually switching off the engine at every red light, level crossing, or short wait , comes with a critical nuance that most people miss.

When It Makes Sense (and When It Doesn't)

Modern factory stop-start systems are designed and tested to handle thousands of short engine restart cycles. The starter motor, battery, and engine management system are engineered for this duty cycle. They restart the engine smoothly in milliseconds and manage the additional wear appropriately.

A standard vehicle not equipped with a stop-start system has components , particularly the starter motor, ring gear on the flywheel, and standard battery , that were not designed for this frequency of use. Manually switching off for 30-second stops and restarting repeatedly will accelerate wear on these components, potentially costing more in repairs than the fuel saving produces.

The Real Threshold

For a non-stop-start vehicle, the generally accepted break-even point is around 60–90 seconds. If you're going to be stationary for more than that , a long freight train crossing, waiting for someone who'll be 5 minutes , switching off makes sense. For a 30-second red light, don't bother.

The myth context: for vehicles with factory stop-start, let the system do its job. Overriding it by pressing the off button defeats the engineering purpose and saves negligible extra fuel over what the system already handles.

Myth 5: Filling Up in the Morning Gives You More Fuel for Your Dollar

This one has circulated in Australian forums and Facebook groups for years: fill your tank in the early morning when it's cool, because petrol is denser when cold and you get 'more' for your litre than you would in the heat of the afternoon.

Why It Doesn't Work in Practice

The theory is technically correct about thermal expansion , liquids do contract when cold and expand when warm. But the application fails for several real-world reasons:

•       Petrol stored underground at servos is insulated from surface temperature variations. The temperature of the fuel you pump at 6am versus 2pm is almost identical , typically varying by less than 1–2°C because it's stored in large underground tanks that don't heat up and cool down the way surface materials do

•       Even if there were a meaningful temperature difference, the density change per degree Celsius is so small that the variation in litres pumped at typical Australian temperature swings would be a fraction of a per cent , unmeasurable at the bowser level

•       Australian fuel is measured and sold by volume at the pump, not by mass. Servo equipment is calibrated to deliver the stated number of litres regardless of temperature

The myth in numbers: assuming a 5°C temperature difference (far more than actually exists between underground tanks at different times of day) and a 60-litre fill: the theoretical 'extra' fuel from cooler petrol is less than 0.1 of a litre. Not worth reorganising your morning around.

Myth 6: Windows Down Is Always More Efficient Than Air Conditioning

The windows-vs-AC debate has been going on for as long as both have existed in passenger vehicles. The 'obvious' answer , windows down is free, AC costs fuel , is only half the story.

The Full Picture

At low speeds , below about 60–70 km/h , in moderate temperatures, open windows are generally more efficient than running the air conditioning. The drag from open windows at low speeds is relatively minor, and the AC system adds meaningful load to the engine.

At highway speeds, the equation flips. Open windows at 100 km/h create significant aerodynamic drag that increases fuel consumption measurably , by an amount that can equal or exceed what the AC system would use. This is why the old advice doesn't hold universally: it depends on speed.

The Practical Rule for Australian Conditions

In stop-start city traffic where speeds are mostly below 60 km/h: open windows in mild conditions save fuel over AC. In sustained highway driving at 100 km/h or above: windows up and AC on tends to be more efficient or approximately equivalent. In extreme Australian heat where maximum cooling is required regardless of speed: the AC is going to be on anyway, and the key is using it efficiently.

•       Use recirculation mode once the cabin has cooled , the compressor works far less hard on pre-cooled air

•       Pre-cool the cabin by briefly opening doors before starting the engine

•       Set the thermostat a degree or two warmer than absolute maximum , 22°C rather than 18°C , and the AC load drops noticeably

Myth 7: Coasting in Neutral Saves Fuel

The logic seems sound: if you put the car in neutral and coast, the engine is just idling , using minimal fuel. Compare this to being in gear where the engine is somehow 'working harder', and neutral seems like the obvious choice for downhill runs and approaching traffic lights.

Why This Is Backwards on a Modern Car

When a modern fuel-injected car is coasting in gear with your foot off the throttle , what engineers call overrun , the fuel injection system cuts fuel delivery almost entirely. The engine is being driven by the transmission and wheels, not the other way around, and the computer recognises this and shuts off injection. You're using close to zero fuel.

When you put the car in neutral while moving, the engine is no longer being driven by the wheels , it's now idling. Idling requires the engine to burn fuel to maintain its own rotation. So coasting in neutral doesn't reduce fuel use to zero; it actually uses more fuel than coasting in gear on a modern vehicle.

The Real-World Implication

When you see a red light ahead or traffic slowing, the most efficient response is to keep the car in gear, lift your foot off the throttle, and coast down to the stop , letting the fuel cutoff do its job. Don't dip the clutch or shift to neutral until you're almost stopped and the engine would otherwise stall.

Safety note: coasting in neutral also reduces engine braking and gives you less control in an emergency. It's worse for efficiency and worse for safety.

Myth 8: Slightly Over-Inflating Tyres Saves Fuel

There's a grain of truth buried in this one, which is probably why it persists. Under-inflated tyres do increase rolling resistance and fuel consumption , that part is correct. And over-inflating tyres does reduce rolling resistance slightly. So the logic goes: inflate beyond the recommended pressure for better economy.

Why the Recommendation Exists

Tyre pressure recommendations are set by the vehicle manufacturer after testing that balances fuel economy, ride quality, tyre wear, and , critically , grip. An over-inflated tyre has a smaller, harder contact patch with the road. Less rubber touching the road means reduced grip, particularly in wet conditions.

Driving on significantly over-inflated tyres in wet weather increases stopping distances, reduces cornering grip, and makes the vehicle more prone to aquaplaning. The tyre also wears unevenly , excessively on the centre tread , shortening its lifespan.

What This Means Practically

The correct pressure is the recommended pressure , not maximum, not 10% above recommendation. The right answer to tyre pressure is precision, not excess. Keeping tyres at their correct recommended pressure (at the low end of a heavy-load recommendation on most days) is where the economy and safety optimum sits.

The economy savings from over-inflation: negligible, and not worth the compromise in wet-weather grip. The economy savings from under-inflation to correct pressure: 2–5%, completely safe, and free.

Myth 9: Hypermiling Tricks Like Drafting Behind Trucks Save Significant Fuel

'Hypermiling' , the practice of maximising fuel economy through extreme driving techniques , has a dedicated following online. Some of the techniques are legitimate (smooth driving, cruise control, reducing weight). But some of the more extreme ones , particularly drafting closely behind large trucks on highways to reduce aerodynamic drag , are both overstated in their benefit and genuinely dangerous.

The Drafting Myth

Drafting , tucking in behind a truck at close range to use its slipstream , requires being dangerously close to the rear of a heavy vehicle. The potential stopping distance needed if the truck brakes suddenly from highway speed is far greater than the following distance required to be in the truck's slipstream. The fuel saving from drafting at a safe following distance is negligible.

Legitimate aerodynamic gains from positioning on a highway come from keeping windows up, removing roof racks, and maintaining a consistent speed , not from tailgating heavy vehicles.

What Legitimate Eco-Driving Looks Like

The real gains in eco-driving come from the techniques covered throughout this blog and elsewhere: smooth acceleration, early throttle lift, maintaining tyre pressure, removing unnecessary weight and drag, and sensible highway speeds. These are safe, legal, and genuinely effective. The extreme techniques , which sometimes include coasting on public roads with the engine off , are dangerous and illegal on Australian roads.

Myth 10: Buying Petrol at Busy Servos Gives You Better Value

The theory here is that high-turnover stations have fresher petrol that hasn't degraded sitting in underground tanks, and this fresher fuel burns more efficiently. It sounds plausible but doesn't hold up to scrutiny.

How Long Petrol Actually Stays Good

Modern petrol stored in properly maintained underground tanks (the standard for all licensed Australian servos) remains perfectly stable for months , far longer than the typical turnover time of even a quiet suburban station. The stability additives blended into retail petrol are specifically designed to maintain properties over extended storage periods.

There is no meaningful quality difference between petrol at a busy city servo and a quieter suburban outlet from the same brand, assuming both meet Australian fuel quality standards , which licensed retailers are required to do.

What Actually Matters When Choosing a Servo

What does matter: the price, the brand (major brands with reputable additive packages versus unbranded or generic), and your timing relative to the weekly price cycle. A quality-branded servo on a Tuesday morning will always beat a premium servo on a Friday afternoon from a value-for-money perspective , station busyness has nothing to do with it.

Myth vs Reality at a Glance

The Myth	The Reality	Actual Money Impact
Premium petrol improves any car's economy	Only beneficial in engines specifying it	Costs $150–$400/yr for zero return
Fuel additives improve efficiency	No measurable benefit in maintained engines	Costs $30–$150/yr with no saving
Warm up engine by idling	Wastes fuel; gentle driving warms faster	Wastes ~$40/yr idling in driveway
Manual engine off at every red light	Increases wear; not designed for frequent cycling	Potential starter/battery cost exceeds savings
Fill up in the morning for more fuel	Underground tanks don't vary with surface temp	Zero benefit , reorganising morning for nothing
Windows always beat AC for economy	Only at speeds below ~60–70 km/h	Wrong at highway speeds , windows add drag
Coasting in neutral saves fuel	In-gear overrun cuts injection; neutral requires idle fuel	Uses more fuel than staying in gear
Over-inflate tyres for better economy	Reduces grip; uneven wear; tiny economy gain	Increases safety risk; negligible saving
Draft behind trucks to save fuel	Only effective at dangerous following distances	Safety risk , legitimate gains require tailgating
Busy servos have better quality petrol	Licensed storage maintains quality for months	No quality difference , just choose cheapest

What Actually Does Work , The Short List

With the myths cleared out, here's what the evidence consistently supports as genuinely effective:

•       Tyre pressure at correct specification , free, 2–5% economy improvement, five minutes monthly

•       Smooth, progressive acceleration and early coasting , free, 10–20% improvement in city driving

•       Highway speed discipline , driving at 100 km/h instead of 120–130 km/h on long runs , free, 15–20% improvement on highway legs

•       Air filter replacement when dirty , $20–$45, 6–11% improvement when filter is significantly clogged

•       Oil and filter service on schedule with correct grade , $80–$180, 1–3% improvement plus long-term engine protection

•       Removing roof racks and boot clutter when not in use , free, 5–25% improvement on highway

•       Buying on the low day of the weekly price cycle using a price comparison app , free, saves 20–30 cents per litre versus peak day

•       Using the correct octane grade , free, 5–15% improvement if previously using wrong grade in a 95-spec engine

The Bottom Line

The most useful thing about understanding these myths isn't just knowing they're wrong , it's understanding what they're costing you. Premium petrol in the wrong engine, regular additive purchases, and daily idling warm-ups can add up to $500 or more per year in unnecessary spending with zero benefit in return.

The real savings are almost always in the things that cost nothing: tyre pressure, driving smoothly, sensible highway speeds, and timing your fill-up to the weekly price cycle. These aren't exciting , there's no product to sell and no gadget to install , but the evidence for them is solid and the financial return is real.

If someone is selling you a fuel-saving product or technique, the first question to ask is whether there's independent testing behind the claim. For the myths covered in this guide, the answer is consistently no.

Frequently Asked Questions

Are any fuel-saving devices actually worth buying?

Independent testing of the vast majority of aftermarket 'fuel-saving devices' , fuel magnets, vortex generators, ionisers, hydrogen injection kits, and similar products , finds no measurable economy improvement in controlled testing. Several of these product categories have been the subject of consumer protection action in Australia and internationally for making unsubstantiated claims. The ACCC has previously taken action against marketers of fuel-saving devices with unproven claims. For money spent on the car, the best returns come from standard maintenance items , air filters, correct oil, tyre pressure , rather than aftermarket add-ons. If a device claims to improve economy by 15–25%, independent verification of that claim under standardised test conditions should exist before you invest. For most products on the market, it doesn't.

Does hypermiling really work, and is it legal in Australia?

Some hypermiling techniques are genuinely effective and fully legal , smooth acceleration, early throttle lift, maintaining tyre pressure, removing unnecessary weight, using cruise control, and reducing highway speed. These are simply eco-driving applied consistently, and they produce real economy improvements measurable across a tank or a year of driving. Other hypermiling techniques are not legal or not safe on Australian roads. Engine-off coasting , switching off the ignition while the vehicle is in motion to eliminate engine braking drag , is illegal in most Australian states as it compromises steering assist and braking systems. Excessive drafting behind heavy vehicles at close range violates minimum following distance laws and is extremely dangerous. The legitimate techniques work. The extreme ones are not worth the risk.

Is it true that filling up more often (smaller amounts) saves money?

This myth suggests that carrying a full tank adds weight and therefore increases fuel consumption, so filling in smaller increments saves money. The reasoning has a sliver of theoretical truth , a full 60-litre tank of petrol weighs about 44 kg, and extra weight does marginally increase fuel consumption. But the maths don't support making it a deliberate strategy. The extra fuel consumption from carrying a full versus half tank of petrol is approximately 0.3–0.5% , a few cents per 100 km at most. Against this, filling up twice as often doubles your exposure to buying at a high point in the weekly price cycle. The potential price-cycle cost (up to 30 cents per litre if you happen to fill on the wrong day) completely swamps the weight-related consumption saving. Fill up on the cheapest day of the cycle regardless of how full the tank already is , that's the strategy that actually holds up.