Because of my recent interest in petrol prices around Melbourne, I've been reading articles and questions about whether or not it is a good idea to fill up first thing in the morning. The idea is petrol expands as it gets hotter. The mornings are cooler so you get more petrol for your buck.
From my observation there is something missing from this conversation. It is how petrol prices change during the day. Each day prices around Melbourne drop by half to one cent. If you buy in the morning you'll buy at a higher price from the day before since prices don't appear to go down overnight.
I decided to see what I could find out. The density of petrol changes about 0.12% per degree C. So yes petrol density does change with temperature. If we look at the temperature change from minimum to maximum, it is unusual to see more than a 10 degree change. The minimum would often be around 5-6 am and the maximum mid afternoon. So even if you did decide to fill up in the morning, most likely it will be after the temperature starts increasing. However this assumes a direct relationship with the ambient temperature. Petrol is stored underground and is less affected by the ambient temperature. For simplicity lets say the temperature did change for say 5 degrees. That would be 0.6% change in volume. Perhaps in summer with hotter days, this may mean a larger temperature variation.
The price of petrol in Melbourne is currently fluctuating between around 100 cents to 130 cents per litre over roughly a four week cycle. With prices in the discount part of the cycle changing around 0.5 - 1 cent per day, filling up later could save half to one cent per litre.
Too put this into perspective, if we fill up a 50 litre tank and there was a 0.6% density change due to temperature, if we use 115 as the price per litre of fuel, that would gives us a saving of 30 cents.
On the other hand, if we fill up at the end of the day and take advantage of the dropping price, that could save us 25 - 50 cents.
In all sincerity I suspect the actual temperature variation would be lower since the tanks are stored in the ground, but in any case, you're more likely to gain more by taking advantage of the dropping price. Overall however, saving 30 or 50 cents, is a very small amount and it would be hard to justify changing your habits for such a small saving. There's bigger and better savings to be made. However if I were to pick, I'd pick filling up towards the end of the day because the price cutting will most likely mean a better saving.
Kelvin Eldridge
www.PetrolPricesMelbourne.com.au
From my observation there is something missing from this conversation. It is how petrol prices change during the day. Each day prices around Melbourne drop by half to one cent. If you buy in the morning you'll buy at a higher price from the day before since prices don't appear to go down overnight.
I decided to see what I could find out. The density of petrol changes about 0.12% per degree C. So yes petrol density does change with temperature. If we look at the temperature change from minimum to maximum, it is unusual to see more than a 10 degree change. The minimum would often be around 5-6 am and the maximum mid afternoon. So even if you did decide to fill up in the morning, most likely it will be after the temperature starts increasing. However this assumes a direct relationship with the ambient temperature. Petrol is stored underground and is less affected by the ambient temperature. For simplicity lets say the temperature did change for say 5 degrees. That would be 0.6% change in volume. Perhaps in summer with hotter days, this may mean a larger temperature variation.
The price of petrol in Melbourne is currently fluctuating between around 100 cents to 130 cents per litre over roughly a four week cycle. With prices in the discount part of the cycle changing around 0.5 - 1 cent per day, filling up later could save half to one cent per litre.
Too put this into perspective, if we fill up a 50 litre tank and there was a 0.6% density change due to temperature, if we use 115 as the price per litre of fuel, that would gives us a saving of 30 cents.
On the other hand, if we fill up at the end of the day and take advantage of the dropping price, that could save us 25 - 50 cents.
In all sincerity I suspect the actual temperature variation would be lower since the tanks are stored in the ground, but in any case, you're more likely to gain more by taking advantage of the dropping price. Overall however, saving 30 or 50 cents, is a very small amount and it would be hard to justify changing your habits for such a small saving. There's bigger and better savings to be made. However if I were to pick, I'd pick filling up towards the end of the day because the price cutting will most likely mean a better saving.
Kelvin Eldridge
www.PetrolPricesMelbourne.com.au
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