fuel economyFuel economy is a major consideration when buying a car.  One of the most common questions car buyers in the UK ask, and certainly one I have been asked many times, is “Should I buy a car with a petrol or diesel engine?”  Well, there’s no simple answer; it depends on a number of factors.  In this blog, we’ll explore this in more detail.

Many car buyers have got themselves tangled up in false economic arguments when trying to choose between a petrol or a diesel engine when buying a new or used car.  The correct choice is very much dependent on your specific circumstances and driving style.  One of the first things that The Car Expert does when working with a client to find the perfect car is to carefully and thoroughly analyse their car usage to get a clear understanding of what their needs are.

For this article, we are not considering hybrid or purely electric cars, because it complicates things massively and will be the subject of a separate article later on, but let’s consider a straightforward example of diesel vs. petrol.

Volkswagen Golf fuel economy - petrol or dieselThe Volkswagen Golf has long been considered the benchmark for mid-size hatchbacks, and is available in a range of petrol and diesel engines with identical specifications.  This makes it a good example to compare the engines.  For this example, we will compare the 1.4 TSI 122hp petrol engine with the 2.0 TDI 140hp diesel engine.  In the real world, these engines give comparable overall performance.  We will compare manual gearboxes and assume the customer has paid full retail price.  Servicing and maintenance costs are broadly comparable between the two models, so the difference will really come down to the engines.

We will use the official urban and combined fuel economy figures as provided by the government (extra-urban figures are usually useless), and assume the buyer keeps the car for three years, covering 10,000 miles per year.  Resale value is provided by WhatCar? Magazine.  Fuel prices are provided by petrolprices.com as of today (5 March 2012).  We have to assume that fuel and registration costs stay at 2012 prices for the next 3 years (if only!) and that insurance costs are broadly similar for each car (which they should be).

Petrol:

  • Golf Match 1.4 TSI 122hp – £19,100
  • Value after 3 years – £7,640 (40% of new car price)
  • Fuel economy, urban – 34.4 mpg
  • Fuel economy, combined – 45.6 mpg
  • Road tax (first year included in new car price) – £130/year

Diesel:

  • Golf Match 2.0 TDI 140hp – £21,090
  • Value after 3 years – £9,491 (45% of new car price)
  • Fuel economy, urban – 46.3 mpg
  • Fuel economy, combined – 58.9 mpg
  • Road tax (first year included in new car price) – £95/year

So the diesel car is more expensive by nearly £2,000 when new, but should be worth about £1,800 more after three years.  It costs less to register (£35/year at present, so not a lot in it) and should use less fuel on average.

Total cost of buying and running each car for three years (excluding insurance, servicing and maintenance) are as follows:

Petrol:

  • Using Urban fuel economy figures – £17,335
  • Using Combined fuel economy figures – £15,964

Diesel:

  • Using Urban fuel economy figures – £16,293
  • Using Combined fuel economy figures – £15,300

So the diesel Golf is £347/year cheaper to run using the urban figures, and £221/year cheaper to run using the combined figures.  In the overall of a £20,000 car, this is hardly a significant difference, and doesn’t take into account individual driving circumstances.  The government fuel economy tests are not really representative of real-world driving, and very few drivers ever match the official figures.  Below I will detail the differences in the way a petrol engine drives compared to a diesel engine.

What can be said for certain is that the higher your annual mileage, the better the diesel becomes.  The above calculation is based on 10,000 miles per year, which is considered average.  If your mileage is higher than that, the diesel car’s advantage is greater.  If your mileage is lower, the figures become even closer.

The above example is also very specific to that car and those two engines.  If you are looking at a different car, then you need to use the relevant figures for the engines available for that model.

What is most important of all is properly considering how you will really use the car for as long as you own it, but here is a very general breakdown of the generic differences between petrol and diesel engines:

  • A petrol engine usually produces more power than a comparable diesel, making it more responsive at very low revs, so pulling away from rest at the lights or at a T-junction is usually quicker in a petrol engine.  A diesel engine will have to work harder and therefore use more fuel to keep up.
  • A diesel engine usually has more torque than a petrol engine, making it better at carrying heavy loads.  So a diesel engine maintains its performance and economy much better when you load up a car with a full load of passengers and a bootful of luggage, whereas a petrol engine will struggle noticeably in comparison, losing performance and using more fuel.
  • A diesel engines tends to be more economical in steady-state driving, such as on a motorway, when the engine is at its optimal point and cruising along comfortably.  A petrol engine will have to work harder to maintain its momentum and will use more fuel.
  • Stop-start town driving is inefficient for any kind of engine, so there is much less difference in fuel economy between a petrol and diesel engine around town.  Especially if you have to work the diesel engine harder under acceleration.
  • A petrol engine produces fewer overall emissions per litre of fuel burned than a diesel engine.  But a diesel engine will usually use fewer litres, so be may more eco-friendly for a given distance (internationally measured in grams per kilometre – g/km).  But that’s assuming optimal conditions, because…
  • Short journeys are very inefficient for any kind of engine, as cold engines use more fuel and pollute a lot more than warm engines.  This applies to both petrol and diesel engines, and means neither engine approaches anywhere near its ‘official’ fuel economy figures.  A journey has to last at least 10-15 minutes before the fuel economy starts to hit its best figure, and prior to that both engines are using lots of fuel.
  • Diesel-engined cars tend to cost more than petrol-engine cars when new.  However, their resale value is usually better (this is very dependent on fuel prices at the time, so is only ever a prediction of what will happen in the future).
  • The sound of a diesel engine is still off-putting to some people, although they are much more refined than they used to be.

More than just fuel economy

We need to take all of the above into account when deciding whether to go for a petrol or diesel car.  Funnily enough, the recommendations provided by The Car Expert are split about 50/50 for petrol and diesel, with factors other than fuel economy often being more important.  Also, if you’re going to buy a used car, there may be very little difference in price between a petrol car and a diesel car.  But the overall message is to carefully evaluate how you are going to use the car before laying down your hard-earned money.

The Car Expert compares real-world fuel economy to official fuel consumption figures

Want to know more about fuel economy?  Read The Car Expert‘s latest article on why your real-world fuel consumption doesn’t match the official manufacturer figures.

Want to know more about the specific issues for using a diesel car for city driving?  Read The Car Expert‘s article on diesel cars and urban driving.

Should you pay extra for premium fuels?  The Car Expert looks at premium diesel and premium unleaded petrol, and whether they’re worth the money.

 

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Stuart Masson is the owner and editor of The Car Expert, a London-based website which provides expert and impartial advice for anyone buying a new or used car, as well as news and information from all over the automotive world.

This Post Has 75 Comments

    1. stuart

      With any older/higher-mileage used car, you need to be careful in checking the overall condition and service history. If the car looks good and has a fully-stamped service book, then the mileage is less of an issue. If it has little or no service history, or the owner has gone to different places anytime it has needed work done, it suggests that the car has not been that well looked after, and you should probably give it a miss.

      Obviously, a high-mileage is going to be cheaper than an identical car with lower mileage, but if you use some common sense and consider the above advice, you should be able to decide if the car is reasonable or suspect.

  1. nicole

    Dear stuart. Your article is great, so thank you. I wanted to ask your advice on whether it is better for me to buy a diesel automatic or diesel manual car, which is an 04 plate, with low mileage? I do long distance driving; about 1,000 per month. Many thanks in advance for your time and expertise. :0)

    1. stuart

      Hi Nicole. If you are doing long drives on motorways and country roads, a manual 2004 car will probably give better mileage than an auto. With newer cars, there is usually less difference in performance and economy between manual and auto. If you are doing more town driving, an auto is certainly more comfortable in stop-start traffic but will use more fuel. stuart.

  2. Nagendra

    can you advice me Stuart, how ever about fule, power and maintenace cost, which engine is better for rare usage means(car will run only 10 to 15 km/10days), under this situation which engine will be more workable.

    With best Wishes,
    Nagendra

    1. Stuart Masson

      Hi Nagendra. If you are talking about new or very modern cars/engines, then there shouldn’t be any problems with either petrol or diesel for occasional use. If you are talking about older cars/engines, then I would lead towards petrol. Older diesel engines can be harder to start and be more difficult to maintain. Best wishes, stuart.

      1. Nagendra

        Hi Stuart, Thank you very much for your opinion, if i want to buy a used diesel engine car for above mentioned condition, can you advice me how many years old car to be suitable. With best wishes, Nagendra.

  3. Stuart Masson

    Hi Stephen. The predicted resale on a diesel Golf is indeed better than on a similar petrol Golf. Used car buyers tend to be more conscious of running costs than new car buyers, so diesel cars are more sought-after and hence command higher prices. Thanks, stuart.

  4. Stuart Masson

    Improving fuel economy and reducing emissions (both heavily related) are the two primary goals of car companies’ engineers these days, and there have been significant improvements to both petrol and diesel engines. We are seeing downsized engines with advanced turbocharging and engine management systems, which make the engines far more efficient. Diesel engine improvements have been facilitated by advances in fuel injection and turbocharging, and these advances have subsequently been applied to petrol engines as well. Diesels have also benefitted from a century of innovation in petrol engines, during which time diesel engines were largely ignored but are now learning lessons from petrol engine development. Both will continue to improve in coming years.

  5. mike fernando

    can you advise me STUART which 10 year old used estate car is the best for 10,000 miles p/a, taking into consideration everything (insurance/ sevicing /parts/ MPG cost). I have a VX astra. the rear leg room is minimal. VERY POOR VENTILATION. Ford Focus? MAny thanks mate. Mike

    1. Stuart Masson

      Hi Mike,

      If you want an estate car, the next size up does a much better job than your current size (Astra). Something like a Ford Mondeo or a Volkswagen Passat – or if you don’t want to go quite that big, the Škoda Octavia is excellent. If you’d like more detailed assistance, check out the pricing and feature information for our different packages here.

      Cheers, stuart

    1. Stuart Masson

      Mike, the diesel residue around the pumps will always be dirtier than the fuel in the car because it accumulates dirt and dust from the air after it has been spilled. The fuel inside the bowser passes into your tank without being exposed to the air so it is certainly much cleaner. The oil which you get all over your hands actually lubricates the cylinder in a diesel engine.

      But it would be good if more service stations kept their bowsers clean!

  6. Alex

    This is dumb. Diesel is for city petrol is for racing and for highway. Why to drive diesel when it cost more for repairing? People still do not understand. I was driving diesel. DIesel is for slow driver. But remember if u want to go faster u need to push the gass and its normaly it will take more gasoline. So this articel is uselles. I got mustang 3.8v6 35mpg. For normal drive city 22mpg. ANd its fun. DIesel is cool when u drive in city 200 kilometers per day. But reparing cost to much. Why they make petrol then ? Imagine This diesel is 2.0 motor and petrol 1.2 WV? Why ferrri dont make diesel lamborgini or buggati?

  7. Roelf S

    It’s amazing how much conflicting information is around the net about this when choosing between a petrol car and a diesel one. Thankyou for clearly explaining what we should be mindful of.

  8. Henry Fitzmaurice

    I have a diesel company car and it’s fine for clocking up lots of miles, but for weekends I have an old Porsche 928, and the engine is brilliant. It gets about 15mpg but I can forgive it because it’s wonderful.

  9. Model Trains

    It really brings the overall impression of a nice car down when it’s chugging like a tractor at the traffic lights. Couldn’t bring myself to drive one.

  10. Felicita Geoffrey

    I read in the Evening Standard the other day that a diesel car will be more economical if you do more than 10,000 miles a year. Is that a good rule of thumb?

    1. Stuart Masson

      It very much depends on the car, the driver and the circumstances, but it’s a reasonable starting point. The example I used above was based on 10,000 miles/year simply because that’s about the average annual mileage for UK motorists. But at The Car Expert, we will always look closely at the client’s specific circumstances to make some more accurate and relevant recommendations. Cheers!

  11. Joe Cool

    Why do petrol engines seem to be used more commonly with hybrids instead of diesel? I would have thought that an electric motor for town driving and a diesel engine for country roads would be a perfect combination.

    1. Stuart Masson

      Hi Joe. It does sound great in theory, but diesel engines have so far struggled with hybrid applications – although this is starting to change. Part of the problem is that most hybrids still require the engine to start and stop fairly frequently, and diesels are not as good at this as petrol engines are. Plus they need additional filters – especially as they emite more pollutants when cold, and are heavier than petrol engines, so the overall car weight gets ever heavier. But they will get there eventually.

  12. Tom Mickelson

    New common rail diesels are the way to go. I used to have a Golf with the old unit pump diesel, and it was the noisest car around. Now my wife has a newer Golf with the common rail engine and its so much quieter I can hardly believe it. I got so sick of driving teh old one, but enjoy trips in the new one.

  13. Nightwing74

    We have two cars in our household. The main car is a Mondeo diesel, and it’s boring but effective. Gets us over 40mpg in mixed city and motorway driving. My weekend car is a BMW E46 M3, and for flinging it down a country road in the sunshine on a Sunday morning it’s unbeatable. Horses for courses, boys and girls.

  14. Tex

    I have always been a petrol fan (American, hence the nickname ‘Tex’ in the UK), but was impressed by the last diesel I drove which was in a BMW 3-series. Would strongly think about it, even in the US where gas is much cheaper.

  15. Fred

    Some of the new diesel engines on the market from BMW, Audi and Mercedes are just incredible. I’d have one over the petrol any day. You have to be a real diesel denier to drive a petrol car these days.

  16. James

    @Edd, I’m with you. Have had three diesels now and simply wouldn’t consider going back to petrol. Even if it’s not really saving me much money, I like not having to fill up as often, so I feel like I’m saving.

    1. Stuart Masson

      Hi jodie. If you are only doing very short trips, it is likely that your annual mileage is quite low (unless you are doing very short trips regularly but not so regularly that the engine stays warm in between, which is unlikely). If so, then it may be that a diesel car is not going to save you money over a petrol car. Or the amount gained may be insignficant enough not to be the deciding factor.

    1. Stuart Masson

      Yes Tex, that is correct. If you don’t sell the car, then the petrol has still worked out cheaper after 3 years. However after 4-5 years (depending on mileage) the diesel car once again noses in front and then increases its advantage each year (assuming parity between petrol and diesel fuel prices).

  17. BMW girl

    My biggest issue with diesels is the noise, they sound like trucks rather than cars. Even if the diesel car was cheaper to buy I would rather have the petrol one. Its worth it.

    1. Stuart Masson

      Hi BMW girl. Yes, diesel engines have traditionally sounded much rougher than petrol engines, although this is changing quite dramatically. Given your name, you may be pleased to know that BMW diesel engines are some of the best around…

  18. PAUL JONES

    sorry can you explain why the petrol Golf is worth 40% after 3 years yet the diesel Golf is worth 45%. Is this just a typo as I can’t see why the diesel car would be worth more PAUL

    1. Stuart Masson

      Hi Paul,

      Good question. The 3-year values are a prediction, based on historical data. And yes, the diesel version of the Golf traditionally holds its value better than the petrol version. This is largely because used car buyers tend to be looking to save money (hence the reason to buy a used car rather than the same car brand new), and favour the diesel model for its lower running costs. In addition to having a higher resale value after 3 years, it is also much better after 1 and 2 years, as the decrease is more linear than for a petrol car (which drops a lot in the first year and then flattens out a bit more)

  19. lv

    Stuart, what’s your opinion on the Miles-Per-Litre vs Miles-Per-Gallon debate? I think that the government is using fuel pricing in litres just to hide their tax grabs – 3p/litre is about 14p/gallon, which most people don’t realise. If they did there would be uproar.

    1. Stuart Masson

      My opinion is that the UK should join the 21st century and adopt the metric system all round ;)
      But I think you make a valid point on the government being content to leave things as they are to make fuel tax increases easier to introduce.

    1. Stuart Masson

      Good question Lucas, but unfortunately not an easy answer. It’s difficult to predict how the prices of petrol and diesel will change in the next few years other than to say that it’s likely that both petrol and diesel will get ever more expensive. Assuming that the price parity between petrol and diesel doesn’t change too much, then the diesel becomes more favourable as they both get more expensive and fuel becomes a greater part of the overall cost of the car.

  20. Jake Brenton Hawley

    It’s about time AMERICA and CHINA embraced diesel. With so many people and such large countries, they could save so much wasted fuel if their cars were diesel and not petrol.

    1. Stuart Masson

      Hi Danny,

      It all depends on the car and how you drive it. Premium diesel fuels can give you slightly more performance (although you’d be hard-pressed to notice the difference) if you drive your car hard, and they can also give slightly better economy. Whether the improvements justify the extra expense is something you’d have to measure for yourself. Fill your car up half a dozen times with regular diesel and note your mileage. Then repeat half a dozen times for the premium diesel. By doing it several times, you will start to see any difference. Just comparing one tank of each may not accurately reflect normal use.

  21. Lee loves petrol

    Death to diesels! You did miss the biggest disadvantage of a derv though, and that’s getting oily disel all over your hands EVERY time you have to fill up. Why can’t petrol stations keep their pumps clean?

  22. Dhiraj Patra

    In your analysis it is based on the price of fuel are almost same for both.

    But if you see for a particular country say in India, Govt give huge subsidies to diesel fuel in thinking (saying to public) that it use most of the public transport vehicle in country still now which mostly use by general people.

    So Govt spending our tax money to giving subsidies for poor or general people as it told.

    In practical situation is different. People are now buying mostly (only) Diesel vehicle for small to big all types of cars so that they can run their vehicle in very very less cost compared to very high price of Petrol (compared to diesel it is about 60% costlier and increasing then and now).

    But I hope the alternate fuel like Hydrogen molecule based can be used in existing petrol engine without much changes.

    But for the time being people tend to buy diesel car will continue at least in 3 rd largest market in world, India.

    1. Stuart Masson

      Hi Dhiraj,

      Thanks for your e-mail. My analysis is based on the price of cars, their resale value, fuel and taxes in the UK as my business helps customers in the UK. The principles will apply equally well in India, however you will need to substitute my numbers for those which apply to your situation to work out if a petrol car or a diesel car is more suitable for your circumstances. Until recently in Europe, diesel was much cheaper than petrol, but taxes have been increased over the last few years to the point where diesel is now more expensive.

      Another factor which favours diesel cars in India is that modern petrol engines are very sensitive to using high-octane and high-quality fuel which is not always available in all parts of the world. Diesel engines tend to be more tolerant to lower fuel quality, so they are less likely to have problems if the standard of fuel available is not as good as it is in Europe.

      Best wishes,

      stuart

      1. Dhiraj Patra

        Yes true. Meanwhile a strong protest taking places in different areas of this continent due to regularly increase in fuel prices.

        Govt, both center and in state level taking huge taxes on fuel. If they cut down the taxes in respect of that fuel is essential commodities then it will definitely come down to 50% of its current price.

        Also mainly due to car companies pressure (as then are using same 100 yrs old technologies with some booster technologies) for current car. And alternate energy like Hydrogen molecular based car need more R&D for them. People force to use the conventional fuel still now.

        In India comparative to income fuel price is tremendously high ($1.5 per liter). And transportation mainly based on it. Electric is also not affluent. So in the mean time we need Hydrogen based alternate fuel for vehicle.

        1. Stuart Masson

          I can assure you that India is no different from most of the world in that governments everywhere take a huge amount of tax from petrol and/or diesel! In the UK and Australia it is now over 50%, I believe, and other countries are very similar.

          Hydrogen is still a long way away as it will take an enormous amount of infrastructure to make it happen, and no-one globally is interested in investing that sort of money.

    1. Stuart Masson

      Hi Jim,

      Predicted resale values are calculated by a number of sources, and are usually done over three years as that it the average length of time that people tend to keep their cars. Finance companies do predict resale values for every month up to 4 or 5 years for their finance calculations, but they generally don’t share this information as it is commercially sensitive to the finance rates they are prepared to offer.

      As a rule, you will lose the most value in the first year, less in the second, less again in the third, and so on. Eventually, after about ten years, the value of the car usually drops away to almost nothing regardless of how well it runs or how much you have spent on maintenance. However, if the car is rare or desirable, then resale values tend to stay much stronger and can eventually increase after many years. Don’t hold your breath waiting, though!

    1. Stuart Masson

      Hi Coppin,

      The same broad characteristics of petrol and diesel engines that I listed will apply to most cars. For specific costs like the ones I provided for the Volkswagen Golf above, you would need to get the relevant information (price, resale value, fuel economy, road tax, insurance, etc.) for the petrol and diesel Ford Focus models you are considering and do the same sort of maths to come up with an accurate estimate.

    1. Stuart Masson

      It depends on the particular car, and also on the sort of driving you do. In quite new cars, the autos are often as economical as the manuals. However, older cars tend to be much more economical as manuals. Research the sort of car you are looking at buying for official fuel consumption figures for a better idea – or use The Car Expert to buy your next car and we’ll do all the research for you!

      Also, if you do a lot of country or motorway driving, a manual is usually better as the car weighs less and you are usually cruising along in top gear anyway rather than changing all the time.

      1. Tiny

        My last car was a H-reg (20+ years old), petrol, automatic, 2.6 litre, E-class Merc. (It was the first production car to feature fuel injection.) According to the owners hand book: the automatic got 8 mpg more than the manual version of the same car. Provided you’re in economy mode, an automatic changes gear at precisely the right rpms. Many manual drivers over rev their cars and waste a lot of fuel making a lot of noise. I find it impossible to believe that a manual could be more fuel efficient than an automatic.

        1. Stuart Masson

          The Mercedes W124 was one of their finest cars, although it was far from the first production car to run fuel injection, which American manufacturers and Porsche, among others, had been running for years. Mercedes cars and their engines have long been optimised for automatic transmissions rather than manual, so it is not that surprising that their autos can get better mileage than their manuals. It may also be influences by gearing and final drive ratios, which are not necessarily the same between auto and manual models. In most cars, however, a manual driver driving carefully can usually return better economy figures than an auto driver. If you drive without any real consideration for economy, an auto can certainly be more efficient than a manual.

  23. Stuart Masson

    Hi Nick,

    Comparing a regular petrol or diesel to a hybrid or fully-electric is more complicated for several reasons. There are not many manufacturers who offer the same car with a petrol (or diesel) engine and a hybrid engine – if they do, there are usually other differences as well rather than just the engine. And no-one in the UK currently offers a car with a choice of normal or full-electric engine, so you are comparing different cars as well as different engines. But I will be covering this in more detail soon, so stay tuned!

  24. Areias6439

    That is my first time I have visited here. I’ve discovered so much interesting stuff in your blog on all the different posts. I guess I am not the one one having all the good reading here! keep up the great work. Thank you

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