10 UK Fuel Myths Completely Busted: From Morning Fill-Ups to Premium Fuel

Myth 1: Fill Up in the Morning for Denser Fuel

The claim: Petrol is denser in the cold morning air, so you get more energy per litre if you fill up early.

The reality: Fuel is stored in underground tanks at a near-constant 10–15°C regardless of external temperature. The thermal expansion of petrol in those tanks during a UK summer day (which rarely gets hot enough to cause significant expansion anyway) is negligible — less than 0.1% even on a warm day. The potential saving from morning fills is effectively zero in UK conditions. This myth originated in the USA and Australia where surface-level fuel storage and extreme temperatures make the phenomenon slightly more relevant, but even there the savings are tiny. Do not drive out of your way for a morning fill — use FuelFinderLive to find the cheapest local station regardless of time of day.

Myth 2: Premium Fuel Always Improves Economy

The claim: Premium fuels like Shell V-Power (99 RON) or BP Ultimate significantly improve MPG for all cars.

The reality: Premium fuels only improve performance and economy in engines specifically designed for high-octane fuel. Modern high-performance and turbocharged engines can advance ignition timing when running on 97–99 RON petrol, extracting more power and sometimes improving economy by 1–3%. For standard naturally-aspirated engines designed for 95 RON (E10), the premium fuel provides no measurable benefit in fuel economy or engine protection. The 10–15p per litre premium for V-Power or Ultimate petrol is only justified if your car's handbook recommends or benefits from 98+ RON fuel.

Myth 3: Supermarket Fuel Damages Engines

The claim: Supermarket petrol is lower quality than branded fuels and causes engine deposits and damage over time.

The reality: All petrol and diesel sold in the UK must meet the European EN 228 (petrol) or EN 590 (diesel) specifications. These are legally enforceable standards covering octane rating, sulphur content, ethanol content, and dozens of other parameters. Supermarket fuel meets these standards identically to branded fuel. The difference is that branded fuels like Shell V-Power contain proprietary additive packages (cleaning agents, lubricants) over and above the legal minimum. For most drivers these additives make no measurable difference. For high-mileage vehicles with significant carbon deposits, branded fuels may offer marginal cleaning benefits — but this is a refinement, not a safety concern. Supermarket fuel does not damage your engine.

Myth 4: Driving with Windows Open Saves More Fuel Than Air Con

The claim: Air conditioning uses significant engine power, so opening windows is always more fuel-efficient.

The reality: It depends on speed. At urban speeds below 50mph, air conditioning uses approximately 5–10% more fuel while open windows add minimal aerodynamic drag — so open windows win at low speeds. Above 60–65mph the equation reverses: the aerodynamic drag from open windows exceeds the fuel cost of running air con. On motorway journeys, use air conditioning with windows closed. In slow urban traffic, open the windows. The crossover point varies by vehicle but is typically around 55–65mph.

Myth 5: Idling Uses Less Fuel Than Restarting

The claim: Turning the engine off and on uses a burst of fuel, making short stops more expensive than idling.

The reality: Modern fuel-injected engines use no extra fuel during a restart beyond what would be consumed in approximately 5–7 seconds of idling. If your stop is longer than 10 seconds, turning the engine off saves fuel. This is precisely why modern cars have stop-start systems that automatically cut the engine at traffic lights and in queues. If your car has stop-start, do not override it — it is saving you money. If your car doesn't, develop the habit of switching off during longer stationary periods (railway crossings, school drop-offs, parking). Note: do not switch off while the engine is still warming up, as cold-start fuel consumption is higher.

Myth 6: Running on Low Tank Saves Weight and Fuel

The claim: Keeping your tank nearly empty reduces vehicle weight and improves fuel economy.

The reality: A full 55-litre tank weighs approximately 40kg. The fuel economy improvement from carrying 20 fewer litres (15kg saved) is roughly 0.3–0.5% — worth about 0.5p per litre in fuel savings. This is completely outweighed by the risk of running low and being forced to fill at an expensive station rather than the cheapest nearby. Running your tank consistently low also risks condensation forming in the fuel tank and can reduce the lifespan of your fuel pump, which relies on being submerged in fuel for cooling. Keep at least a quarter tank and use FuelFinderLive to fill at the cheapest station when convenient.

Myth 7: Red Diesel Is Just Dyed Normal Diesel

The claim: Red diesel (gas oil) is identical to road diesel, just with a dye added to identify it for tax purposes.

The reality: Red diesel is chemically similar but not identical to road diesel. It is formulated for off-road machinery and has different sulphur specifications and additive packages. More importantly, using red diesel in road vehicles is illegal — it attracts a lower rate of fuel duty (approximately 11p/litre versus 52.95p for road fuel) and HMRC carries out regular roadside checks. Vehicles found with red diesel in road tanks face fines and vehicle seizure. Since April 2022, the eligibility for red diesel was further restricted, removing access for most commercial sectors except agriculture, fishing, and some rail applications.

Myth 8: Fuel Additives Dramatically Boost Economy

The claim: Bottles of fuel additive sold at petrol stations and online can improve fuel economy by 10–20%.

The reality: Independent testing by the Consumers' Association and motoring organisations has consistently found that aftermarket fuel additives deliver minimal to zero measurable fuel economy improvement in real-world driving. Some cleaning additives do remove carbon deposits from injectors after extended use — which can restore economy that has been lost over time — but these are maintenance products rather than performance enhancers. Modern fuels already contain the additive packages necessary for optimal injector cleanliness. Save your money on additives and spend it on finding cheaper fuel with FuelFinderLive.

Myth 9: E10 Petrol Damages All Older Cars

The claim: Since E10 petrol became the standard grade in 2021, all older cars will suffer fuel system damage.

The reality: E10 petrol (containing up to 10% bioethanol) is compatible with the vast majority of petrol cars built after 2000. DVLA estimates that approximately 95% of petrol cars on UK roads are E10-compatible. Vehicles that are NOT compatible include classic cars built before 1990, some motorcycles and mopeds, and a handful of models from the early 2000s. You can check your specific vehicle at gov.uk/check-vehicle-e10-petrol-compatibility. If your car is not compatible, E5 Super Unleaded (97+ RON) remains available and is E10-free, though at a price premium of approximately 15p/litre.

Myth 10: You Must Fill to 100% Every Time

The claim: Always fill your tank completely to minimise the frequency of stops and maximise convenience.

The reality: Filling strategy should be driven by price optimisation, not convenience habit. If FuelFinderLive shows a significantly cheaper station near your workplace than near your home, it may make more sense to fill a partial tank near home to reach work, then fill fully at the cheap station. Similarly, if prices are forecast to fall (monitor FuelFinderLive's price trend graph), filling partially now and topping up when cheaper can save money. The only caveat is that consistently running your tank very low risks pump damage and emergency fills at expensive stations — so maintain a sensible buffer.