Petrol vs diesel – which fuel is cheapest in the long run?

The cost of servicing isn’t much more with diesels than petrol’s in my experience, though the more cramped engine bay of the diesel should mean higher labour costs in theory. The fuel filter, a serious piece of kit in a diesel will need to be changed more often though.

But Brian has it right, a lot more to go (very expensively) wrong in the modern diesel. Not that it necessarily will. Failed turbocharger (now fitted to all diesels) seem to crop up a lot, often caused by missed or late oil & filter changes. Insist on a synthetic oil come service time & get it changed once a year, or better still, every 6 months. And if you do it yourself (check Halfords online for right grade)you can be sure you get the oil you’ve paid for in your sump.

How do people who have to park in the street cope with charging electric cars? Not really practical to have a flex going across a pavement overnight to your car all night from your house..and that’s if you can find a space outside your house.

Wavechange, it’s my belief that manufacturers reccommended oil & filter changes are set by marketing men not engineers. The Haynes manual for my Fiesta TDCI reccommends every 6 months or 6,000m. Ford say every year or 12,500m..You can get 5L of 5W/30 synthetic oil for around £20 if you look around.

A basic service for a diesel car shouldn’t cost much more than on the petrol version..it’s I’d leave the fuel filter on my diesel to a garage every time. And a full tank of derv in cold weather helps prevent condensation in the tank & subsequent water contamination.

From what I’ve read, if you are planning on doing mainly short journeys around town, a new diesel car may not be a good choice due to the diesel particulate filter fitted to them now. If they don’t get hot enough to burn off the soot (passive regeneration) the car goes into ‘active regeneration’ mode which can lead to engine oil being diluted by fuel..

I could well be being paranoid over water in diesel if you’ve not had any problems all that time Wavechange. No reason a diesel should be more or less reliable than a petrol I’d have thought, apart from no H.T. leads to get damp. The cold starting probs of older diesels seem to have gone what with high pressure common rail injection & better fuel additives. Mine started first time from cold @ an indicated -7.8c back in Feb, though it turned over noticeably slower!

Many car problems these days seem to be caused by electronic glitches, probably the best advice is to check the Which brand reliability pages.

Full marks to the VW man for giving you the honest, rather than corporate information about oil changes.

Just another thought on electric cars. They’re going to be cheap to run (if not to buy) as domestic electricity is subject to 5% VAT, whereas around 65% of the cost of petrol & road diesel is tax. If electric cars became more affordable & practical, would any government allow that imbalance to continue?

Well, we have just made the decision to replace our 2nd car, a BMW120d auto with a newer 120i petrol version. Why? because the 2006 diesel car is always used on short journeys of 10 – 15 miles, work, shopping school runs etc. All long trips are in our new 520d touring ( at best 42mpg by the way) . The 120d with full BMW history averages 31 mpg. That’s about all we can expect. A newer efficient dynamics version (we borrowed one) achieves 40 mpg. The 120i on the other hand is a 44 mpg petrol car, suited to short journeys, and on loaning a demo I achieved a staggering 42 mpg driving really hard.
Fuel efficiency of petrol cars especially the new BMW’s has improved so much that when you take the differential in price between petrol and diesel into account, and the poor economy of many diesels on short trips (especially in winter) it’s a no brainer – petrol wins. We also paid £2k less for the same spec car than if we had gone for an equivalent diesel version. So, lower cost, lower pollution, lower noise pollution and a quieter drive. We are very happy to now be able to drive away from the pumps not smelling of diesel, and 50p per gallon better off!

So a 7p difference in a roughly £1.40 overall cost (so, 5%) means diesel is suddenly so much more expensive than petrol that you will lose money by using it? Er… ok… let’s just quickly think this one through before we go writing a whole article about it.

Let’s say there’s an 8p difference, and the two fuels are £1.36 and £1.44 (4p either way of 1.40)… If your petrol car gets 34mpg (or, 25 miles to the pound), how efficient does the diesel one have to be to be exactly as cost effective? 25 x 1.44 = … 36mpg.

Two mpg better.

If your diesel car is only 2mpg better than the equivalent petrol, something’s gone badly wrong.

Currently I’m driving a diesel which – at about the same speeds on almost exactly the same route as I drove to and from work a year ago in an equivalent petrol model – gets 54mpg vs the old car’s 34mpg. Once you factor in the per-litre price increase (effectively reducing the diesel economy to 51mpg, on a fairly taxing commute), I can drive 50% further for each pound spent. In other words, it’s cut a third off my fuel costs. And I think the engine is probably not running anywhere near as well as it should, either.

It cost me about £400 extra vs what a like-for-like replacement for the written-off petrol would have (£2400 vs £2000), and driving roughly 7500 miles a year (~£1250 petrol, ~£840 diesel) I’ll be about ten quid in the black once it hits its first anniversary. £420 up by its second.

Really, this was a bit of a nothing idea as the difference between each fuel’s cost per litre when compared to the different in economy is so minimal that you can more or less ignore it.

Auto fuels are an emotive issue because of the high cost in proportion to our incomes. Nevertheless, they play a key role in enviromental issues and one that must be addressed in a clear, sensible way. Changes will not be implimented overnight, whatever some people might want/believe.

Auto diesel is more effective (I did not say efficient) for reasons that have already been described above, most notably the higher energy content per weight and the fact that the diesel engine operates at higher compression ratios which makes in more thermodynamically efficient.

However, there are growning concerns over the health issues surroundin the combustion process of diesel. Accelerating levels of micro polutants in our cities (ALL cities) is direct consequence of the increase in diesel-engine vehicles. Put simply, petrol requires a very precise mixture of air-fuel ratio in order to ignite and when it does burn it is sudden and explosive. Diesel on the other hand will burn whatever the air ratio is. Less air and it burns poorly, producing vast quantities of unburn and partially burnt fuel. Because of the combustion characteristic, even the best-maintained and designed deisel engine produses a range of these particules durin gthe combustion cycle. YOu have heard of increasing use of “Particle Filters”, but this is only a method of dealing with the problem, not removing the problem. Petrol engines, especially the newer ones, are more effective in their combustion process but produce more carbon because they burn more fuel (same weight but more volume.

In essence, all fuels are hydrocarbons. We have gone from using coal, pure carbon, to oils a range of which have a varying proportion of hydrogen to carbon. Auto diesel contains slightly more carbon than gasolene (petrol), however, gas (LNG, LPG) has even greater hydrogen content than carbon. The move towards gas away from oils is a step in the right direction as it moves more toward the hydrogen end of the hydrocarbon scale.

However, we have to expose these “renewable” fuel oils for the sham they are. Vegetable-based oil stocks are even more carbon loaded (because they come from plants) but the real issue is the vast areas of the world’s vegitation that is being cleared in order to produce these so-called “renewable” oils tha tin the end are just as polluting as fossil fuels. It has the added (and worrying) feature of removing the world’s oxygen-producing “lungs” and hence reduces the ability of the world’s atmosphere to recover from natural polluting events.

Bear this in mind…… In it’s life a tree absorbs a vast amount of carbon from the atmosphere. When you cut down that tree and burn the wood, all that lifetime of carbon is then released. Any oil from the tree (or it’s fruit) contains the carbon absorbed in that time of growing and is subsequently released. Also, if a treee falls and lays there rotting, the decaying process also releases all the carbon trapped in the cells. Head they win, tails you lose. All the carbon gets release whatever way the tree’s wood is disposed of.