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Petrol vs Diesel

Last Updated on Monday, 23 October 2017 10:36

Petrol vs Diesel – Which is cheaper to run?

In recent years the popularity of diesel engines has steadily increased to the point where sales of diesel cars now slightly exceed those of petrol cars.

The introduction of turbo diesels and fuel injection technology has significantly improved the performance of diesel engines. In addition, you typically get more MPG from a diesel (and therefore less CO2 emissions) and with rapidly increasing fuel costs and the introduction of a CO2 based taxation system, diesels are also now considered cheaper to run.

But are diesels really more economical and do their environmental credentials stack up? While you will get a greater MPG from a diesel car, diesel fuel is about 5 pence per litre more than unleaded petrol. We’ve done some calculations of our own on three of Britain’s top selling cars, the Ford Fiesta, the Nissan Qashquai and the BMW 320. We have compared the annual fuel and tax costs of the diesel and petrol versions of these cars. We selected identical engine sizes and specification for each model. The only difference is the engine/fuel type.

The calculations are based on the official MPG figures published by the manufacturers. Whilst we acknowledge the limitations of these official figures, there are no alternatives available and they do at least ensure like for like comparison.

For each of the three different manufacturers/models we have calculated the annual car tax and fuel costs of the petrol and diesel models at an annual mileage of 10,000 miles, 15,000 miles and 20,000 miles. The calculations use the current prices for unleaded petrol and diesel fuel. The results shown in the tables below are quite surprising.

Annual car tax costs

Car tax varies according a car’s CO2 emissions. The higher the CO2 emissions, the higher the tax. For each of our three chosen cars, the petrol versions fall into tax band D (£100 per year) and the diesel versions fall in to band B (£20 per year). In each case there is a saving of £80 per year on car tax with the diesel engine.

Annual fuel costs

For all three cars the diesel engines are much cheaper to run, and the greater the mileage the greater the savings in fuel costs. What is interesting is that the annual fuel costs and savings are very similar for all three cars. At 10,000 miles annual mileage the diesel cars are just over £400 a year cheaper to run, at 15,000 miles the savings are about £580 and at 20,000 miles the savings on fuel costs are almost £750 for the diesel engine cars. Although diesel fuel is about 4% more expensive than petrol, you get approximately 35% - 40% greater mpg in a diesel car. Therefore the more miles you do the more money you save.

Ford Fiesta Edge 1.4 Petrol vs DieselAnnual Mileage and Fuel/Tax costs
Car Tax (Band)MPG (Combined)10,000 miles15,000 miles20,000 miles
Ford Fiesta 1.4 Petrol£100 (Band D)49.6 £ 1350 £ 1975 £ 2600
Ford Fiesta 1.4 Tdci Diesel£20 (Band B)70.6 £ 936 £ 1394 £ 1851
Annual savings with diesel £ 414 £ 582 £ 749

Qashquai Acenta 1.6 Petrol vs DieselAnnual Mileage and Fuel/Tax costs
Car Tax (Band)MPG (Combined)10,000 miles15,000 miles20,000 miles
Acenta 1.6 Petrol£100 (Band D)45.6 £ 1460 £ 2140 £ 2820
Acenta 1.6 dCi £20 (Band B)62.8 £ 1049 £ 1564 £ 2079
Annual savings with diesel £ 410 £ 576 £ 741

BMW 320 Petrol vs DieselAnnual Mileage and Fuel/Tax costs
Car Tax (Band)MPG (Combined)10,000 miles15,000 miles20,000 miles
BMW 320i SE£100 (Band D)44.8 £ 1484 £ 2176 £ 2868
BMW 320d SE£20 (Band B)61.4 £ 1073 £ 1599 £ 2126
Annual savings with diesel £ 411 £ 577 £ 742

Don’t diesel cars cost more to buy?

Diesel cars are typically more expensive to buy than the equivalent petrol model. This was the case for all three of the cars we evaluated. The price differential ranged from an extra £795 for the Fiesta 1.4 diesel to £2200 for the BMW 320 diesel. However this additional purchase cost is retained in the value of the car. Indeed it is generally considered that diesel cars depreciate less than petrol cars, particularly for high mileage cars. We looked at the current second hand valuations for 2009 registrations of the petrol and diesel versions of the three cars in our study. For all three models the diesel models had depreciated less than the petrol models. This is therefore an additional saving for diesel engines. For this reason we do not believe purchase price is part of the equation when comparing fuel economy of diesel and petrol engine cars.

Petrol Vs Diesel - which is cheaper to run?

Despite the higher price of diesel fuel, diesel cars are still cheaper to run than petrol cars due to their much superior fuel economy. The higher the mileage you do, the greater the savings. There is also a saving on car tax.

As the price of fuel continues to rise, fuel economy is becoming an increasingly important criterion in a car purchasing decision. If you are considering the petrol vs diesel versions of a particular car then it is worth investigating the fuel savings for the mileage that you do.

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0 # Petrol vs DieselColin Hamp 2015-06-28 08:33
In the article you assert the difference in purchase price is not a factor. This is clearly false. As illustration, I have bought my last three cars new and kept them to the end (scrapped). Clearly my whole life cost includes the purchase price, the useful working life and all finance, running and maintenance costs. I believe a diesel may have a longer service life and that should be factored in. My last pertol car failed to reach 150,000 miles, one of my current diesels is on the way to 250,000
verdict - 'Could do better'. Would you re-do your sums and re-submit?
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0 # petrol vs dieselTim 2012-11-10 17:52
And if you want to throw electric cars into the mix.

Electri c cars typically use about 300Wh/mile, which at 12p/kwh, gives 3.6p/mile or £360 per annum for 10,000 miles, unless you are on Economy 7 at 6p/kwh, which gives £180 per 10,000 miles.

It is not surprising that electric cars have a very limited demand. Some hybrid cars may eventually be just as cheap to run as pure electric cars. The plug-in hybrids look like the best bet for future needs, with the engine disconnected from the driving functions altogether.

Cheaper to run?

A plug-in hybrid doing 10,000 miles total, 5,000 miles of "electric only" and 5,000 miles of normal driving might manage £90 electric and £300 LPG or £400 diesel.

Perh aps we are getting to the point of returns becoming fully diminished, where spending £25,000 on an electric car just isn't worth the investment if you can buy another £25,000 car that can do anything with no worries over range.

If the depreciation over 100,000 miles is 25p/mile, saving 5p per mile on fuel is not so significant. Will we ever get cars that can do 5p per mile on fuel, but only cost £5,000 to £10,000? Or will they only be possible for people who run cars up to 200,000 miles or more.

Cheape r to run?

When will the capital costs be taken into account?
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0 # petrol vs dieselTim 2012-11-10 16:43
And if you want to do a real world mpg and cost comparison, don't use the Combined MPG, only use the URBAN figures, and apply a +10% correction factor.

So if the Urban MPG figure is 30, then the real world will be about 33mpg, unless you are on motorways all day and every day.

If you have run a car and measured your own real MPG and compared it with the manufacturers, then you might be able to work out your own more accurate correction factor.

The extra-urban should be taken as the maximum possible fuel economy!
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0 # petrol vs dieselTim 2012-11-10 16:27
This is a very limited comparison.

If you want efficiency, you have to take capital cost into account, after all, the cost of the car in the first place represents an expenditure of energy by the car manufacturer on R&D and production.

If you want cheaper to run, you have missed the most important analysis of all.

Taxi drivers who are going for the highest efficiency, cheapest to run, are using Toyota Prius on LPG.

When buying a car, you not only have the MPG, but also the cost of the fuel, AND maintenance costs.

The Toyota Prius has very low maintenance costs, the lowest in the automotive world. You might get the odd exception, a bad battery on one car, or another failure, but statistically they are the most reliable, and have the lowest maintenance costs.

Doing 50 mpg in a Prius at 70p/l, is much lower cost than doing 70mpg at 130 or 140 p/l, it gives about 30% lower fuel costs.

There fore, if you had a Prius on LPG at 10,000 miles per annum at 50mpg costs £636
A Toyota Prius on LPG at 10,000 miles per annum at 60mpg costs £530
or on LPG at 10,000 miles per annum at 70mpg costs £454 (if you are able to adapt your driving style)

Petro l vs Diesel is an old comparison from years ago. The automotive world has moved on from there!!

I currently run a petrol 1.8turbo, 29mpg average, £1000 per annum on LPG. But not all modern petrol engines can run on LPG. Some manufacturers are going down a bit of a blind-alley R&D route, which might give increased fuel economy, but will give unreliable cars with poor residuals over the long-term.
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