From shelf-life to mixing fuels – here's everything you need to know about petrol

·9-min read
fuel petrol diesel - image/graphic
fuel petrol diesel - image/graphic

Most drivers usually have, on average, half a tank of fuel in their car. One result of the recent panic-buying means that a greater proportion of drivers will no longer need to fill up over the coming days and, along with an improvement in the supply of fuel from refineries, the ‘crisis’ will peter out.

However, while Britain’s drivers are in a heightened state of anxiety about the supply of fuel, there remain a number of questions, particularly about how the introduction of the new E10 petrol might affect them.

And, if you’re struggling to find petrol or diesel, we’ve included some fuel-saving tips to help you eke out the precious contents of your tank. These also save you money should fuel prices remain high.

Does petrol expire?

Petrol will degrade over time but in the sealed tank of your car (or fuel station forecourt) it will remain fresh for at least six months. This rate of degradation varies depending on the number and type of additives in the petrol, which will deteriorate at different rates.

So rest assured that even if the current fuel scare drags on throughout the winter, any fuel in your car will still be good until next spring.

A BP petrol station that has run out of fuel is seen in south London - Toby Melville/Reuters
A BP petrol station that has run out of fuel is seen in south London - Toby Melville/Reuters

Even then, it won’t be ruined, although as fuel degrades it leads to poor starting and uneven running – but it won’t wreck your engine.

Can I mix E5 and E10 petrol?

Since the start of September, the new E10 has been the regular grade of unleaded petrol in the UK. This denotes a 10 per cent content of bioethanol distilled from plant material.

E5 (a 5 per cent concentration of bioethanol) was the previous standard blend of petrol. It’s unlikely there’s any left at filling stations but your car’s tank may contain some if you filled up in late August and haven’t used your car much in the subsequent month.

If you mix the two there is simply not a problem.

Can you mix unleaded and superunleaded?

Yes. All current cars are designed to run on standard 95 octane unleaded (even high-performance cars) but will run equally well – if not better – on the higher octane superunleaded, which has an octane rating of 97 or above.

In essence, it provides a bigger bang although it costs significantly more than regular unleaded due to a high concentration of additives including detergents to keep your engine clean.

Incidentally, superunleaded fuel remains E5 while the 95 octane regular grade contains up to 10 per cent bioethanol.

What happens if you put the wrong petrol in your car?

There’s no such thing as the wrong petrol, as you can mix regular and superunleaded, E5 and E10, in any ratio and you simply won’t notice any difference.

Much more likely is accidentally filling with diesel. If you’ve done that, don’t attempt to start the car and call out the emergency services or a garage as the fuel system will have to be flushed.

What about diesel?

Diesel fuel is also in shorter supply than normal. As with petrol, it will last for at least six months without going “off” although some suppliers reckon that it will last up to a year depending on storage conditions.

Diesel is also in short supply - GettyCont
Diesel is also in short supply - GettyCont

The varying proportions of additives make the rate of degradation variable, but it simply isn’t an issue.

What is the best way to store petrol?

You can store up to 30 litres (that’s about half a tankful) at home.

Petrol is highly volatile (diesel less so) and if you really must store some at home it needs to be in a strong, sealed container to avoid spillages and also ensure that the fuel doesn’t go “off”.

Purpose-designed plastic containers will suffice but here at Telegraph Cars we’re sticklers for a stout metal jerry can. If it’s good enough for the Army, it’s good enough for us.

It might seem obvious, but fuel should be kept out of direct sunlight and away from all sources of heat or ignition.

10 tips to save fuel

1. Make fewer trips

This seems obvious, but did you know that when you drive a car that has been parked for a few hours, the engine is cold and it uses much more fuel for the first five miles or so? Ideally you'd combine all your daily errands into one big trip. Often that's not possible if you have to pop out during the day to drop off and collect the family, but try not to go out separately to do the supermarket shop or buy a newspaper.

2. Don't drive at peak times - and, if you have to, drive smoothly

There are few worse places to spend your time than being stuck in a traffic jam, but it's also a very expensive way of travelling. Every time that you stop and start in traffic, your car needs first gear and a huge amount of fuel to get moving again. Second gear is not much better. The best solution is to not travel during the rush hour.

When you stop and start in traffic, you tend to use up a lot of fuel very quickly - Getty
When you stop and start in traffic, you tend to use up a lot of fuel very quickly - Getty

Of course, many of us are still working from home due to the coronavirus outbreak but others still have to commute at particular times of day. But even if you have to drive in the rush hour, you can still save some fuel by trying to understand what the traffic is doing in front of you, and travelling steadily at a slow speed, rather than constantly accelerating and braking.

3. Close the windows (and sunroof, if you've got one)

It's not so much of a problem when you're driving in town (see above), but when you're out of town or on the motorway and moving more quickly, the shape of your car is very important. Car designers call it aerodynamics and make lots of effort to reduce the 'drag' and make the car as sleek as possible. Anything that makes wind noise as your car goes along is actually making your car more expensive to run. You can't do much about the design of your car, but you can avoid making it worse by not leaving the windows and sunroof open. It's better to use the air vents for most of the year, and the air-conditioning when it gets too hot.

4. Remove the roof rack or ski box

This is just like leaving the windows open, but worse. Even if the roof rack is empty, it increases drag and makes your car use more fuel, while a big ski box is like having another car strapped to your roof. The latest roof racks and ski boxes are quick and easy to fit and remove, so make the effort to stow them away when you're not using them.

Exclude any excess weight, such as a roof rack or ski box, this will help to lighten the car and ultimately use less fuel - Getty Images
Exclude any excess weight, such as a roof rack or ski box, this will help to lighten the car and ultimately use less fuel - Getty Images

5. Don't carry unnecessary weight

Just like your body, your car needs more fuel to move around more weight. So, just as you wouldn't wear a heavy rucksack unless you had to, don't cart stuff around in the boot of your car unless you need it. Ironically, the heavier the item (the usual culprits are golf clubs and trolleys), the less likely you are to bother taking it out of the boot and the greater the effect it will have on your fuel consumption. You can also reduce weight by filling up with less fuel, more often. You'd be surprised how much more a full tank of fuel weighs than half a tank.

6. Accelerate smoothly

The perfect way to travel is at a constant speed (ideally around 50mph), and in the highest gear. So if you're a patient driver, you'll have lower fuel bills - it's as simple as that. It's unrealistic to avoid overtaking, but there's little point accelerating past a car to simply be in front of it at the next set of lights - any instant gratification will appear on your fuel bill the next time you fill up.

7. Don't press the accelerator down too far

This one always surprises people. It's not just to do with what gear you're in. You may be in a high gear and travelling at a sensible speed, but if you're pushing the accelerator down a long way to avoid changing into a lower gear (into third from fourth, for example) then you're actually using more fuel, not less. Obviously, if your car has an automatic gearbox, then it will probably do a better job than you of choosing which gear to be in, so it's not a problem.

8. Turn off the air-conditioning

Air-conditioning tends to use a lot a fuel - Getty
Air-conditioning tends to use a lot a fuel - Getty

It's tempting to leave the air-con on all year round. It stops the windows misting up in the winter and you don't ever need to think about the temperature inside the car, but it uses quite a bit of fuel, so we'd advise you turn it off when it's not hot (on most cars the button has a snowflake symbol). Don't forget to run it every couple of weeks to keep the system lubricated and in good condition, though.

9. Stick to the speed limit

If you ignored the law, you could shave a bit of time off your journey by travelling above the speed limit, particularly on long motorway trips. But, although you might arrive about 20 minutes early on a 200-mile trip by travelling at 80mph instead of 70mph, it's also a false economy. While the car is running for 20 minutes less, it uses much more fuel when it is travelling. That 20 minutes could cost you as much as £7 extra in fuel.

10. Check your tyre pressures regularly

The lower the tyre pressure, the more fuel the car needs to move it down the road. We recommend that you take five minutes every fortnight to check the tyres. If you're not sure what the pressure should be, you can normally find the figures near the lock inside the driver's door.

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