Powertrain
Diesel, petrol or electric — doing the sums
8 min read
The question has no general answer, but it can be calculated. Four quantities decide it: your annual mileage, your driving profile, whether you can charge at home, and how long you intend to keep the car.
The four quantities
Before any powertrain is a candidate, you need your own figures. They are in the service book and on the last inspection report — mileage divided by the years since first registration gives your actual annual mileage, and for most people that is lower than they guess.
- Annual mileage: under 10,000, between 10,000 and 25,000, or above.
- Driving profile: mostly short trips in town, or mostly motorway.
- Charging at home or at work — yes or no.
- Planned ownership period. Under three years, depreciation dominates everything else.
When a diesel pays
A diesel uses less fuel, costs more to buy and more to maintain — the particulate filter, exhaust gas recirculation and injection system are items a petrol car does not have. It only recovers that extra cost over distance. The rule of thumb has for years been a threshold around twenty thousand kilometres a year; below that it rarely pays.
The second point is the driving profile. A diesel used mostly for short trips never properly clears its particulate filter — regeneration needs operating temperature over a sustained period. A city diesel is therefore not just uneconomical but fault-prone.
When an electric car pays
The decisive question is not range but charging. If you can charge at home you pay the household tariff and drive very cheaply. If you depend entirely on public fast chargers you pay a multiple of that — and the sums look completely different.
Buying used adds a second factor: the state of the battery. It is the single biggest determinant of value and it can be read out. A car with no evidence of battery health is a gamble — on the most expensive component in the car.
What is regularly forgotten
Most comparisons weigh consumption against consumption and stop there. The items that actually make the difference sit further down the calculation.
- Depreciation. On a young car it is the largest single item and usually exceeds fuel costs by a wide margin.
- Insurance. Ratings differ considerably between powertrains and between models — get a quote before you buy.
- Vehicle tax. Depending on country and region it is assessed on engine size, weight or CO₂, and the differences are large.
- Tyres. Heavy cars — which most electric cars are — wear them faster, and the sizes cost more.
- Upcoming servicing. A timing belt change or a major service due shifts any calculation.
And the hybrid?
A full hybrid without a plug is economical in town and needs no charging infrastructure — for short trips often the most sensible choice. A plug-in hybrid, by contrast, only pays if it is actually charged regularly; uncharged it carries a heavy battery around and uses more than the comparable petrol car.