Buying a car is hard enough when there are hundreds of different models to choose from, but these days one of the hardest decisions to make is whether to go for petrol or diesel. Diesels aren’t the slow and smoky chariots of the old days; modern diesel engines are powerful, refined and don’t send out black clouds, but does it make sense for you?

New or used

The first thing to consider when choosing between diesel or petrol is whether your next car will be new or used. If you’re buying used then the decision is a little easier; unless you have a particular distaste for diesel or drive very low mileages (less than 6,000 miles per year) then a diesel makes a lot of sense.

Diesel engines tend to be tougher than their petrol equivalents, and on balance are better able to cope with higher mileages.

Also the price difference between a used petrol and diesel car gets smaller with age, so you’ll benefit more from the better fuel consumption.

But if you’re buying new, there’s a bit more homework to be done.

Now versus later

If you’re choosing between a new petrol car and its diesel equivalent, you’ll have noticed that the diesel model is more expensive; the price difference can be as little as €500 or even several thousand euros. On the one hand it makes some sense to go for the cheaper version, but there’s more to it than that.

Although your diesel car will be more expensive to buy and the fuel itself is more expensive, it will use less of it. You can expect as much as 25 extra miles per gallon of fuel from a diesel if you drive it correctly, and in a year that could mean buying 450 fewer litres of fuel. Drive more than 10,000 miles a year and the savings go up, but drive less than that and you save less.

How you drive matters too. If you do lots of short journeys and few miles, the difference between economy for petrol and diesel will be smaller; diesels take longer to warm up and be at the their most efficient. Many small city cars only come in petrol form for this reason.

Selling already

It might seem like an odd thing to do, but even when buying a car you have to consider when you’ll be selling it. The longer you keep a car the less it is worth, but as a general rule, a diesel car holds its value better than a comparable petrol (unless it is a performance model). So your extra outlay at the start will also result in a better price come selling time.

The bottom line is that you need to do your homework when buying a car. Start with your budget, what you need it for and how far you will drive in a year. Only then can you make realistic comparisons between petrol and diesel and decide what makes the most sense for you.