Motors FAQ>Buying>Petrol or diesel

Since the introduction of emissions based road tax and VRT in July 2008, the petrol vs diesel conundrum has become much more difficult to fathom. There is no definitive answer due to the number if factors involved.

This guide should clear up some of it, by explaining the differences between petrol and diesel cars, and the costs/savings for each and hopefully get you thinking about your own situation and how it applies to you.

The money thing.
Our motivation to change car can come from many directions. Newer and shinier will always appeal, safety and reliability will appeal to some, and saving money will make the decision seem sensible.
Saving money is a great way to sell the idea to yourself. Cheap tax and promises of 60mpg can take the sting out of spending a large chunk of money.

Cost of ownership.
The cost of owning a car can be divided into 3 areas - Buying, Running and Selling.

Buying: The price you pay for the car in the first place. If you're financing the car, the interest on this is added to this price.
The cost of buying a new diesel car has reduced dramatically since 2008.
For example, back then the price premium of a diesel Avensis was €5000, now it's only €500.

Running: What you spend on road tax, fuel, servicing, repairs

Selling: what the car is worth after a certain time.

It goes without saying that the depreciation in the first year of a cars life is the greatest. Many a sour-grapes rant started with "you lose €2000 as soon as it goes out of the showroom".
The figure that really matters is how much the car depreciates up to the point that you sell it.
Nobody has a crystal ball of car values, but past performance can give a hint towards future returns. Look at 5 year old examples of the new car you're looking at to get a rough idea of how they depreciate.
It should be added that diesel used cars still command a premium over petrols.

How significant is "Running", really?
Once you do the maths on Buying and Selling figures, you'll see that there are large sums of money involved. Depreciation is likely to be your biggest cost.
To make this up, you'll need to save money on Running. This is where the cheap tax and big MPG comes in.
It should be noted that manufacturers claimed MPG figures are wildly optimistic, there are many sites out there that show true mpg gathered from drivers. Don't bank on fuel savings. Your own car will have a much better fuel official consumption figure than you're getting now.

Significance of journey types.
The type of driving you should be a major deciding factor in what type of engine you choose.

If you're doing a lot of long distance driving and high mileages, a diesel is the obvious choice - the low fuel consumption and useful torque will help you chew up the miles with less cost.

If you're doing mainly short journeys and low mileage the benefits of diesel aren't as apparent. All engines use more fuel when they're cold, and a diesel takes longer to warm up than petrol.
More significant is the fact that modern diesels are now fitted with a DPF (Diesel particulate filter). This device collects the soot that you would have seen belching from an old diesel car, particularly when the engine was cold - when the unit reaches a certain temperature it burns off this soot. This process usually takes 15-20 minutes, so you need to be doing a weekly open-road trip of at least
30 minutes to prevent the filter clogging up. Replacement of a DPF is a major expense, and would easily outweigh any savings made on fuel.

Alternatives to diesel
For those seeking lower running costs, there are a raft of alternatives out there including electric cars, hybrids and turbocharged petrol cars.
These can often prove better suited to low mileage and urban driving conditions.
There are also a number of LPG conversion companies who will convert your car to use this cheaper fuel.

Conclusion
You may now be more confused than when you started reading this, but by sitting down and working out your current running costs vs your possible running costs on your new car, the buying price minus the selling price once its time to change again, and the type of driving you do - you can figure out whether a new car makes financial sense, and which fuel is best for you.