Diesel or Petrol?

With the price of fuel and emissions taxes at record highs, choosing the best type of car is more important than ever. Often one of the most important decisions is about the type of fuel you want to use.

The most common choice is between diesel and petrol (although you may also wish to consider electric, hybrid or lpg powered vehicles)

The choice between petrol and diesel is never a simple one. Before we go into detail, here's a potted history.

Oil crisis of the 70's

In the US in particular, sub-compacts (as European sized cars were often described), became very popular. Motorwary speed limits were reduced and fuel economy was the most important factor. Diesel cars, though fuel efficient, were rarely promoted in the US, mostly because they were regarded as very polluting due to particulates (still a very important negative factor). Diesels were very popular in mainland Europe due to low cost of fuel and low consumption but not so popular in UK due to very low levels of performance.

90's onwards
Fuel economy seemed to go out of the window, Europeans discovered SUV's for town driving, and 4 litre engines became the norm. Admitedly,all cars were much more fuel efficient (size for size) than before, but outrageous levels of power and performance were desirable. Diesels increasingly popular in UK due to improved performance (thanks to superchargers and turbochargers, and higher fuel economy).

2008
Suddenly the cost of motoring hits home ... worldwide. Sales of SUV's plummet and resale values of large thirsty cars drop like stones. It's not green that's important, it's the cost of living. Diesels account for higher proportion of sales (50% in UK) , but cost of diesel is very high due to limited worldwide production of diesel fuel compared to petrol.

Choosing between diesel and petrol

It's definitely not an easy choice, certainly in the UK, probably in Europe. Here are the pros and cons.

Pros

Economy - Diesels are generally more economical with fuel under most driving conditions. They tend to produce consistently high fuel economy regardeless as to how they are driven. Petrol fueled cars become much less economical when driven agressively. You'll notice that a high proportion of the most economical cars are diesels. You can check vehicle fuel economy rankings here.

Durability - Diesel engines are inherently stronger and diesel cars tend to last longer (although the bodywork may go before the engine)

Lower CO2 emissions - Diesels have lower CO2 emissions, however they are more polluting than petrol in terms of particulates

Diesels have a higher resale value, this may offset the higer purchase price in the long term.

Cons

Cost of purchase - Diesel cars are more expensive to purchase, unless you do more than a certain number of miles you may not make an equivalent saving on fuel. According to the Automobile Association a diesel family car costs on average £1,400 more than its petrol equivalent and usually takes more than 45,000 miles before this cost is recouped. See the article at thisismoney.com here.

Pollution - Diesels (especially poorly maintained ones) producte a lot of particles (hydrocarbons) and these can be a health hazard. This particulate pollution was one of the reasons that adoption of diesels for private transportation was slow in the 70's in the US.

Verdict

It's not possible to give a clear cut verdict on which type of fuel will suit you best. But some simple guidelines are given below.

Diesels will not prove to be cost effective unless you intend to cover quite a lot of miles in your vehicle before you change it. Typically you will need to cover about 45000 miles to break even.

The cost of fuel is only one part of the equation if you're trying to mimize motoring costs. Road tax, servicing, depreciation and cost to insure are very significant costs. Overall fuel economy may not be important to you if you do very low mileage.

New cars are more efficient (and your old car may have deteriorated compared to new model) but the cost of new, and the extremely significant cost of depreciation may make it more economical to run your old vehicle into the ground (not an option if being green and non-polluting is a very important factor to you).