When it’s time to buy your next car, there’s an important question you should ask yourself first - petrol or diesel. Why? Because whether you buy a car with a petrol or diesel engine will affect the running cost of the vehicle.
It’s not a simple case of one being more expensive to run than the other, because many other factors need to be taken into account. When a manufacturer offers both petrol and diesel variants of the same car, here are some points to consider when making your mind up about which one to go for.
Cost to buy
Usually diesel cars are more expensive to buy than equivalent petrol cars. This is because the size and design of diesel engines mean that more parts are required to make them. To burn diesel efficiently some engines also need a turbo, and certain diesel engines also have an electric heater so they’re easier to start in cold weather - both of these also add to the cost.
This isn’t always the case, so check the prices of equivalent models to see which works out better value.
Cost of fuel
It's important to understand that just because one type of fuel is cheaper than the other, it doesn't mean it's cheaper to run your car on the fuel that costs less.
A diesel engine has improved economy largely due to the higher compression ratio they have compared to petrol engines, which leads to improved thermal efficiency. Because diesel engines tend to be more economical than petrol, the difference between the fuel prices when diesel is more expensive than petrol may be compensated in most cases.
Here’s a quick example. If a diesel car costs £40 to fill up and a petrol car costs £35 for the same amount of fuel, the diesel fuel is 14% more expensive. However, if the petrol engine gives you an average of 50mpg fuel consumption and the diesel engine returns an average of 57mpg, the diesel engine is 14% more efficient than the petrol engine. This means the cost of fuel over the life of the car effectively balances out and the extra you spend on the fuel results in more miles because of the improved efficiency.
It’s worth doing the calculations for the model of vehicle you’re looking at to get a good idea of what the overall return is. Bear in mind though, that although the fuel economy offered by the car may not change much, the difference between the price of petrol and diesel fuel may change over time, so remember to take that into account.
Cost of tax
Usually diesel cars are cheaper to tax than equivalent size petrol engine cars. However, recently it has been discovered that diesel cars produce a high level of air pollutants compared to petrol. Several studies into these emissions have revealed some potentially worrying health implications.
New legislation is due to be introduced in the coming years to force manufacturers to vastly improve the particle emissions of diesel engines.
It used to be the case that diesel engines were noisy and not as smooth as petrol ones. This is not necessarily any longer the case thanks to advancements in design and manufacturing techniques. Nevertheless it might be worth test driving both variants if you’re unsure to get a good idea of the differences between them.
Diesel engines normally provide maximum torque at lower revs, which means you can change gear up quicker. This makes for a more relaxing drive. Some petrol cars, especially those tuned for performance, can be more sporty and engaging to drive compared to their diesel counterparts.
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