I've read a number of articles on the internet about whether or not half filling your petrol tank saves money or not. Obviously visiting the service station twice as often costs time, but let's just focus on the cost of petrol. In every article I read I noticed there was something missing. The petrol price cycle which has a restore point followed by discounting. That is the price hikes significantly over a couple of days and then in the coming weeks drops in price. The graph of the typical Melbourne petrol price cycle can be seen at www.PetrolPricesMelbourne.com.au. Since the price cycle has not been considered I thought I'd write a little on this.
Savings due to weight of petrol
First in terms of saving money. The reason people feels it saves money to half fill your tank is petrol has weight and if you increase the weight of your car it uses more petrol. But the question is by how much. I'll use my purchasing behaviour to illustrate.
I purchased a tank roughly each fortnight. I let the car usually go to empty and then fill up. Filling up takes roughly 50 litres. By the way I'm not really on empty. My car, a Toyota Avensis has a 60 litre tank, so I still have 10 litres left in the tank when I fill up.
If at the start of the period I put in 50 litres and end up using that so have zero of the 50 litres left at the end, then the average number of litres I have throughout the period is 25 litres.
If I only fill up half the amount, or 25 litres then I have to do this twice. The average volume is 12.5 litres.
Thus the difference is 12.5 litres. Petrol weights around 0.74 kg/l (0.71 to 0.77 kg/l) and thus the amount of extra weight averages out to be 9.25 kg.
We need to simplify the maths so we'll assume (as we only need an estimate) the relationship between weight and petrol usage is linear. That is double the weight and twice as much petrol will be used. In my case my car weighs 1,490kg. Yes there will be one or two passengers and some extra things we carry around, but lets just use the weight of the car. We then get 9.25/1490 which is 0.6% A small car is around 1,000kg. The tank is usually smaller, but if we use the same 9.25 we got 0.9%.
The saving will thus be somewhere around 0.5-1%. A tank of petrol costs me around $60, so the saving by filling up twice as often would expect to be around 30 to 60 cents a fortnight or about $7.80-$15.60 per year. That's not really a particularly good saving.
Savings due to petrol price cycle
A second saving can occur if you use the petrol price cycle. This however means filling up completely when the price cycle is at the lowest, and then half filling your tank as the price is dropping. Since the price is dropping by half to one cent per day, every time I half fill (which would be every week after the first two weeks) I would save 25 (litres) by 3.5-7 cents, or if I waited until I was on empty and then half filled (25 litres after two weeks) the price would save 87.5-175 cents. With typical petrol price cycles being roughly monthly, we'd get about 24 (12 x 2) half tank fills and 12 full tank fills a year. The savings value of the half tank fills would thus be $21-$42 per year.
Overall savings
The overall savings from half filling up are not great, but they do exist. Savings from less weight and the petrol price cycle means a possible saving of $28.80 to $57.60 per year.
If you add these savings to buying at the lowest point in the petrol price cycle and shopping around for the lowest price (without going out of your way) and using discount vouchers when appropriate, the savings can add up to many hundreds of dollars a year. That money is better in your pocket than someone else's. However do keep in mind all of this takes time. If time is more important to you than the money, then that's your choice.
If you need help keeping an eye on the petrol price cycle and thus the best time to buy petrol, or even where if you're in the Manningham area, visit www.PetrolPricesMelbourne.com.au.
Kelvin Eldridge
www.PetrolPricesMelbourne.com.au
Savings due to weight of petrol
First in terms of saving money. The reason people feels it saves money to half fill your tank is petrol has weight and if you increase the weight of your car it uses more petrol. But the question is by how much. I'll use my purchasing behaviour to illustrate.
I purchased a tank roughly each fortnight. I let the car usually go to empty and then fill up. Filling up takes roughly 50 litres. By the way I'm not really on empty. My car, a Toyota Avensis has a 60 litre tank, so I still have 10 litres left in the tank when I fill up.
If at the start of the period I put in 50 litres and end up using that so have zero of the 50 litres left at the end, then the average number of litres I have throughout the period is 25 litres.
If I only fill up half the amount, or 25 litres then I have to do this twice. The average volume is 12.5 litres.
Thus the difference is 12.5 litres. Petrol weights around 0.74 kg/l (0.71 to 0.77 kg/l) and thus the amount of extra weight averages out to be 9.25 kg.
We need to simplify the maths so we'll assume (as we only need an estimate) the relationship between weight and petrol usage is linear. That is double the weight and twice as much petrol will be used. In my case my car weighs 1,490kg. Yes there will be one or two passengers and some extra things we carry around, but lets just use the weight of the car. We then get 9.25/1490 which is 0.6% A small car is around 1,000kg. The tank is usually smaller, but if we use the same 9.25 we got 0.9%.
The saving will thus be somewhere around 0.5-1%. A tank of petrol costs me around $60, so the saving by filling up twice as often would expect to be around 30 to 60 cents a fortnight or about $7.80-$15.60 per year. That's not really a particularly good saving.
Savings due to petrol price cycle
A second saving can occur if you use the petrol price cycle. This however means filling up completely when the price cycle is at the lowest, and then half filling your tank as the price is dropping. Since the price is dropping by half to one cent per day, every time I half fill (which would be every week after the first two weeks) I would save 25 (litres) by 3.5-7 cents, or if I waited until I was on empty and then half filled (25 litres after two weeks) the price would save 87.5-175 cents. With typical petrol price cycles being roughly monthly, we'd get about 24 (12 x 2) half tank fills and 12 full tank fills a year. The savings value of the half tank fills would thus be $21-$42 per year.
Overall savings
The overall savings from half filling up are not great, but they do exist. Savings from less weight and the petrol price cycle means a possible saving of $28.80 to $57.60 per year.
If you add these savings to buying at the lowest point in the petrol price cycle and shopping around for the lowest price (without going out of your way) and using discount vouchers when appropriate, the savings can add up to many hundreds of dollars a year. That money is better in your pocket than someone else's. However do keep in mind all of this takes time. If time is more important to you than the money, then that's your choice.
If you need help keeping an eye on the petrol price cycle and thus the best time to buy petrol, or even where if you're in the Manningham area, visit www.PetrolPricesMelbourne.com.au.
Kelvin Eldridge
www.PetrolPricesMelbourne.com.au
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