diesel vs petrol cartoon

Last week Saturday, I went browsing for a new car. And as I left the house, my mother informed me in a serious tone that I should “try and get a diesel”. My mother is a wise woman – and I’ll explain why at the end of this post. But usually, I look at the diesel decision with some cynicism.

The Usual Arguments In Favour Of A Diesel Car

  1. Diesel cars hold their value better than petrol cars.
  2. Diesel cars are more efficient than petrol cars.
  3. Diesel is cheaper than petrol (in SA).
  4. Diesel servicing intervals are less frequent.
  5. Business Insider had some other reasons. But I think they’re mostly bogus. “The price premium over a gasoline car is lower than for a hybrid or electric car.” Honestly.

The Other Important Point

  1. Diesel cars are more expensive than petrol cars.
  2. Diesel services tend to be more expensive than petrol services.

So the question really is: at what point is the diesel car a better deal?

The Math

So I gave it some thought, and what we’re really trying to calculate is mileage. That is: at what point does the fuel efficiency saving become more valuable than the diesel premium. In an equation:

Total Cost of Vehicle = Purchase Price + Running Costs

Initially, the total cost of a petrol vehicle is less than that of a diesel (because running costs are 0 when you first buy your car). But over mileage, the total costs of the petrol vehicle catch up (Running Costspetrol > Running Costsdiesel).

diesel versus petrol

I worked with three cars:

  • The Polo Hatch 1.6 Comfortline
  • The Ford Focus 2.0 Trend
  • The Ford Ranger (the 2.5 XL vs the 2.2 MP XL – it was a bit more difficult to find a like-for-like with the more commercial-type vehicles)

And the mileage calculation

Break-even mileage = (Price Premium × 100) ÷ ([ed.fd] – [ep.fp])


    • ed = fuel efficiency of diesel model
    • fd = fuel price of diesel
    • ep = fuel efficiency of petrol model
    • fp = fuel price of petrol

And here are some answers:

Screen Shot 2014-04-01 at 9.32.55 AM


Which says three things:

  1. If you upgrade your car at around the 60,000km mark, then you should be largely ambivalent about whether you go petrol or diesel.
  2. If you drive distance, you should go diesel.
  3. And if you want a 4×4, you don’t really have a choice.

Maternal Wisdom

In October 2013, President Mugabe gave in on his long-opposed position to mandatory ethanol blending of fuel. Zimbabwe’s parliament forced through legislation introducing 10% mandatory blending of unleaded petrol with the ethanol produced by a single company: Green Fuel. A quote from an official at Zimbabwe Energy Regulatory Authority:

“Anyone caught selling unleaded petrol [from October 2013 onwards] will face criminal prosecution.”

You’d think that the blending might have reduced the petrol price. It has not.

But more importantly, I recently had the opportunity to take my mother’s petrol car for a long distance spin. After the first 150 km, I was down to half of a 50 litre tank. That’s a fuel consumption of 17 litres per 100km, from a car that generally uses only 10.

Which makes it a pretty sweet deal for Green Fuel:

  • a government-contracted monopoly on ethanol production;
  • charging an unleaded price for blended fuel; and
  • the fuel blend drives everyone’s consumption up.

My take-home message: when in Harare, go diesel or go home stay home.