While it seems a little pointless for our mechanics to answer this question (because no one has ever asked), it’s a part of buying a car that can have a big impact on your ownership that most people don’t really consider (or really understand).
So, which is better?
It really depends on each particular use case though, more often than not a diesel car will have better fuel economy, behave better on the highway and be easier to drive in traffic. However, more often than not a diesel car is not our preferred choice.
While diesels are a very fuel efficient motor, they’re also incredibly dirty to run, take a long time to properly heat up (often 10–15 minutes) and are a lot rougher than their petrol counterparts.
The biggest issue is actually the dirtiness of the motors. Because diesel is a different fuel, it also emits different pollutants (particularly NOX and particulate emissions) which mean they have to be fitted with additional emissions equipment (such as exhaust gas recirculation and diesel particulate filters) which only operates properly once the car is hot.
This means, for the majority of people living in the city, these vehicles will never become hot enough for the emissions control systems to operate properly meaning they can become blocked or damaged, an expensive fix.
In addition to this, diesels are often designed to take oils with special properties to reduce the dirtiness of the motor while running, meaning that services are often a fair bit more expensive.
Though there are some real detriments to diesel cars, they’re not without their use cases either. For vehicles that will travel long distances, carry loads or tow often, there’s no better motor (currently) than a diesel. Which is why every truck is powered by a dirty great diesel motor.
So how can you decide whether to buy petrol or diesel?
1. Ask yourself how long you spend in your car each day and how often you drive on the highway. If most of your driving is inner city and 10–15 minutes each way, buy a petrol (or consider electric).
2. Think about whether you tow or carry heavy loads often. If you rarely have anything in your car, there’s really no benefit to getting a diesel.
3. Consider the environment. While diesels are more fuel efficient, the also tend to emit far more pollutants (again why they need better emissions equipment).
In our opinion, if you don’t have a specific reason to buy a diesel car, the best choice is to buy a petrol car. Or better yet, consider whether a hybrid or electric car will fill your need.