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One for the mathematicians


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Ok, so Petrol is cheaper than diesel, by about 12p/litre on average, or roughly 50p/gallon in old money.

 

So can someone come up with a formula to calculate when it is cheaper to run a diesel versus petrol engined vehicle?

 

For example a petrol engine returning 30mpg versus a diesel doing 40mpg?

 

As we were all duped into buying into diesel engines, it may be the time to have a look at petrol again :good:

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Based on 120 mile trip,

 

Petrol = 4 gallons @ 90p litre = 4.086 gallon = 16.34

 

diesel = 3 gallons @ 100p litre - 4.54 gallon = 13.62

 

saving £2.72 per 120 miles.

 

Based on the mileage I do which is about 360 a week I save £8.16 a week or £424.32 a year. Based on an average new car being about 2grand dearer with a diesel engine it would take me about 5 years to start saving money.

 

Mind you I drive a Disco doing 30mpg and I don't know what the petrol versin does but you could probably halve that.

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stuart,

 

The cost per mile is equal to the price per volume divided by the fuel economy of your car. In math terms,

 

£/mi = price/mpg

 

What you really are looking for is the right combination of fuel economy and fuel price to make your petrol cheaper than the diesel. In math terms,

 

price (diesel)/mpg (diesel) > price (petrol)/mpg (petrol)

 

Since you know your fuel economy (30 mpg, 40 mpg), you're looking for the right combination of price that will make the ratios equal. Let's set the price of petrol as x. Then the price of diesel will be x + 0.12. Substituting, we get

 

(x +0.12)/40 > x/30

 

Simplifying the above, we can see:

 

(x+0.12)/x > 40/30

 

Let's substitute back for a second into words.

 

Price of diesel/price of petrol >4/3

 

That means when the price of diesel is 1.33 times as expensive a the price of petrol, you'll break even. Given your constraint of diesel being 0.12p more expensive, we can solve for the price of x:

 

30 (x + 0.12) > 40 x

 

3.6 >10x

 

x<.36

 

So basically, when the price of petrol drops below 0.36 p per liter, you'll be better off driving the petrol. In the meantime, your situation says that the diesel will be cheaper.

 

If you were getting 35 mpg in the petrol and 40 mpg in the diesel you'd be equivalent in either car at current prices. For other price and economy points, just compare the ratio of the price of fuels (price diesel/price petrol) and compare that to the ratio of the ratio of economies (mpg diesel/mpg petrol). If the economy side is bigger than the price side, then diesel is cheaper. If the price side is bigger than the economy side, then petrol is cheaper.

 

 

Thanks,

Rick

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the other factor is that you generally have to service diesels more regularly.

 

My diesel has a routine service every 18,000 miles? Haven't had a petrol for a few years but I can't recall that it's any less for petrol?

 

I do recall my old petrol mileage which was 33 mpg. I get 59 mpg out of a diesel all round (works out about 40-45 urban, up to 70-75 motorway).

 

Current local garage charges are 89.9p per litre for petrol which is £4.09 per gallon, giving 12.39 pence per mile. £2479 per year for an annual mileage 20,000

 

98.9 per litre for diesel or £4.50 per gallon, giving 7.76 pence per mile. £1525 per year for an annual mileage of 20,000.

 

Difference of £953 per year. However, my annual mileage has dropped considerably so when I do swap cars in the future, I'll be looking at the maths more closely.

 

There's no doubt in my mind that diesel drivers are now getting well and truly shafted by the oil companies when it comes to diesel prices, how can something that used to be 10p a litre less now be 10p-12p a litre more to produce? I know that there is such a thing as supply and demand, but there's also such a thing as greedy money grabbing oil companies! (I've just done a quick calculation based on diesel being 10p a litre cheaper than petrol and the annual saving would have been £1250!)

 

Rhiannon

Edited by RhiannonBW
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Thanks all - food for thought.

I take Alex's point about servicing (my diesel is 6,000 mile intervals) so I guess it depends which one you have.

It's a bit of a no brainer when diesels are doing 50+mpg compared to petrol engines of old, but there are some pretty efficient engines out there now, not normally 4x's as they tend to be fairly large lumps.

 

There is no question we are being shafted, there is nothing special about diesel it must cost a fraction of what petrol costs to produce.

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isn't it being taxed harder, becase that fat heiffer that runs the country realised he could get away with it as diesels still seemed cheaper due to the economy. Oh and also that diesels were increasing in popularity.

 

I used to run a 1.2 petrol clio to commute in and that did 50mpg seemed to run on fumes, certainly the cheapest car I've ever run.

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Thanks all - food for thought.

I take Alex's point about servicing (my diesel is 6,000 mile intervals) so I guess it depends which one you have.

It's a bit of a no brainer when diesels are doing 50+mpg compared to petrol engines of old, but there are some pretty efficient engines out there now, not normally 4x's as they tend to be fairly large lumps.

 

There is no question we are being shafted, there is nothing special about diesel it must cost a fraction of what petrol costs to produce.

 

 

Disco td5 engines are 12000 miles service interval .

Harnser .

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Bear in mind that Deisels cost several thousand pounds more than the petrol equivalent.

Up until recently that meant that you didn't get any payback for up to 8 years in some cases.

That will all change for a brief window of about 8 months when dealerships struggle to shift stock in the recession.

Now might be a good time to consider buying a diesel if you are prepared to haggle yourself a deal.

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I'm thinking diesels are a good buy if you're buying something that you're going to keep for a few years, or up to 150,000 miles. If you're someone who wants a new car every 1-3 years then petrol is probably a cheaper option.

 

We were planning on a new car in June, when my current one is 3 years old, but we're now thinking we are going to just keep it until it wears out, so probably another 3-5 years. No point changing it whilst it still does the job it was bought for.

 

Rhiannon

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The choice between petrol versus diesel is part economic and part driving style. Diesels are easy to service, cheap to run (my Audi 2.0TDI gets 55mpg), and have great low end grunt if you pull a boat/caravan.

 

I miss the high revving petrol engine when you want to get some pleasure from driving but love the diesel economy when I get 650 miles from a tank.

 

Horses for courses I think

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Stuart, keep your existing motor - it works fine, is cheap to run and it owes you nothing.

 

The whole diesel vs petrol thing has been a massive mug off. It was an interesting proposition but only when diesel was well over 10p a gallon cheaper.

 

Unless you do a bundle of miles a year and I reckon that would be 15k plus then I doubt there is much in it over say a 5 year period (taking into account purchase price, consumption, servicing and price of petrol vs diesel).

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.... and I reckon that would be 15k plus then I doubt there is much in it over say a 5 year period (taking into account purchase price, consumption, servicing and price of petrol vs diesel).
Spot on, I worked it out on a spreadsheet once and the breakeven point came at about 18K per year over three years, including servicing and everything else I could think of, and that was when diesel was cheaper than petrol.
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