Best time to buy petrol: do prices change in the day, best time to get a fill up - petrol and diesel density

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Various factors - including how hot it is outside - affect how much you get for your buck

Did you know that the time of day that you choose to fill up your tank at the petrol pumps can affect how much fuel you get for your money?

It may only be a small difference - a few less pennies for a few more dribbles of unleaded or diesel - but at a time when fuel prices are again on the up, every little helps. Right?

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That’s according to Martin Lewis, who has said that while rumours of filling up at night getting you more fuel “is a slight urban myth”, people who visit petrol stations in the evenings will receive a "tiny bit extra". And drivers should never fill up their car at 5pm...

The money saving expert revealed that though “the difference is minuscule – pennies at best,” filling up at night when outside temperatures are lower could get you “a tiny, tiny bit extra.”

How does it work?

It’s all to do with how petrol and diesel pumps are calibrated. Most fuel pumps are calibrated to dispense fuel based on volume, specifically the volume of the liquid at the ambient temperature.

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The key factor here is that liquids, including fuels, expand and contract with changes in temperature. When it's colder, the fuel is denser, meaning that for the same volume, you're getting more mass of fuel.

Since the pump measures fuel in volume, you end up receiving a slightly higher mass of fuel than you would at warmer temperatures.

Additionally, since you're getting more fuel molecules per volume, you're essentially getting more energy potential per unit volume of fuel. This can translate to slightly better mileage or performance, though the effect is usually marginal.

How much could you save?

It's essential to note that the difference in fuel volume due to temperature changes is usually quite small and may not be significant for everyday driving; it’s probably not worth holding off on that fill-up until colder weather arrives.

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In terms of petrol, its density typically varies by about 1% for every 8°C change in temperature. So, an approximate 11°C decrease in temperature could result in a density increase of around 1.3%.

For a 50-litre fill-up, a 1.3% increase in density due to temperature change could result in obtaining an extra 0.65 litres of fuel. At £1.50 per litre, this extra 0.65 litres of fuel would save you around £0.98 per fill-up.

Of course, factors such as the design and material of your fuel tank can also affect how much fuel you can actually take in.

So while you might get a tiny bit more fuel for your money during colder temperatures, it's not something you'll likely notice dramatically in practice.

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Do prices change throughout the day?

Petrol prices at UK forecourts can change throughout the day, as fuel prices are influenced by various factors such as global oil prices, currency exchange rates, demand, and local market competition.

As such, many petrol stations adjust their prices multiple times a day to remain competitive and reflect changes in the wholesale cost of fuel.

Some petrol stations may also implement dynamic pricing strategies, where prices are adjusted based on factors like time of day, day of the week or even individual station traffic patterns.

As for what the best time to get the cheapest prices is, the answer to that can vary depending on various factors, but there are some general trends and tips to consider.

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Prices tend to be slightly lower midweek compared to weekends when demand may be higher due to increased travel, and prices may be lower during quieter times of the day, such as early morning or late evening.

Prices can sometimes be higher during major holidays when there is increased demand for travel. You can use apps or websites that track petrol prices to compare prices at different stations in your area and identify when prices are lower and find the best deals.

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