Frugal Motoring – Should I buy a diesel?

Well, no, obv… Except yes?

What started as a rant post, I shall turn into a mini-series.

In Frugal Motoring I will discuss how to cheaply purchase cars, the pros and cons for various purchasing methods (straight up cash, loan, PCP, lease), ongoing political/ government motoring related machinations and how to keep your car running.
This week I will discuss why and when you should purchase a diesel instead of a petrol.

Diesel’s bad rep

As discussed in the last Frugal Motoring post, diesels have a bad rep. Audi continues to push the slide into mediocrity with the recent news that even new build A6/A7 cars are affected (1). But this does not extend to all manufacturers. Many (Mazda/ Merc/ Ford/ Kia) continue to produce diesel vehicles with low emissions and good MPG. The diesel brush tars them unfairly, suggesting all manufacturers have ignored the NOx as particulate emissions problems. If we take a look at AutoExpresses list of best ‘green’ cars for 2018, there slap bang in the middle is a diesel Astra (2).

While if we look at the website Next Green Car, there’s a huge raft of diesels with low emissions. It’s important to note as well that most of these are general family cars, not flash executive motors sporting the latest tech. The technology for low emissions has trickled down to the mainstream (3).

Changes, changes

Taxation is being increased on new cars, and some of the new rules will only affect future vehicles. Additionally, depending on emissions the extra taxation could be as little as £20/year, but are more likely to be around an extra £100-200/year for new low emissions models. Residuals for diesels remain strong but are expected to drop in the future with increased taxation. Governments decreasing taxation on diesels seems unlikely (4).

Recent technology advances by Bosch also claim to mean lower emissions in the future, with a new tech targeting the pesky problem particulates and NOx:

When is diesel a good thing?

Diesel engines continue to offer generally better MPG when running at moderate RPM for long periods. So if you’re doing lots of long distance journeys diesel is probably best.

Diesel also offers better torque at low RPM. This means if you are regularly making long journeys or towing then diesel is a better fuel choice.

Which? offers an excellent calculator to work out how much you could save with efficiencies of a diesel engine:

But it’s worth remembering most of these efficiencies rely upon turbos being spooled up, engines being warm and DPFs operating properly. Short journeys will kill Diesel Particulate Filters, and by nature most diesel engines will struggle to get up operating temperatures doing stop-start work in traffic. Around town this will also mean more invisible pollution for local pedestrians and residents.

Diesels also generally continue to cost more to purchase. This means that unless you’re doing starship mileage you may not save on fuel cost what you initially paid out extra in engine choice. Most buyers will only see a diesel recouping it’s cost after 6-7 years. The new taxation system is set to worsen this.


Broadly, diesels are better for:

  • Towing
  • Long distance commutes or frequent long distance journeys
  • Heavier vehicles
  • Multi-stop journeys where the engine remains warm

If you think this fits you, consider that new diesels will be cheaper to tax and more efficient, but will have a payback time calculation to make. Second hand with have less of this payback due to depreciation, but will be taxed more. Both may see a future drop in residuals. As with all big purchases think carefully about your requirements before buying.

Have a great week,

The FIRE Shrink

Further reading:





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