An electric motor, a petrol engine and a diesel engine, measured on efficiency

Most comparisons of the three start with running costs and purchase price. Both move with tax policy and fuel duty, which differ by country and change by budget. The physical differences do not, and there are three of them.

Efficiency: where the energy goes

A combustion engine is a heat engine, and a heat engine throws most of its fuel away as heat. The US Department of Energy puts a conventional petrol car at roughly 16 to 25 per cent of the fuel's energy reaching the wheels. Diesel does better, in the region of 20 to 30 per cent, because it runs at a higher compression ratio and burns a denser fuel.

An electric drivetrain is not a heat engine. The Department of Energy puts battery electric cars at over 77 per cent of the electrical energy drawn from the grid reaching the wheels, once charging losses are counted.

That gap is not incremental. Three quarters of the fuel in a petrol car is warming the air, and that is the physics, not a tuning problem.

Two honest caveats. This is tank-to-wheel, so it says nothing about how the electricity or the fuel was produced; on a grid running on coal the well-to-wheel picture narrows considerably. And an electric car regains some braking energy, which a combustion car cannot, so the real-world gap in traffic is wider still than the cycle figures suggest.

Torque: where it arrives

A combustion engine makes almost no torque at rest. It has to be spinning, which is why it needs a clutch to get moving and a multi-speed gearbox to stay in its usable band. A diesel's band sits lower and narrower than a petrol engine's, which is why diesels pull well from low revs and run out of breath early.

An electric motor makes its maximum torque from zero rpm and holds it across a wide range. That is why almost every electric car has a single-speed reduction gear and no clutch at all: there is no band to keep it in. A range extender keeps a combustion engine on board but never connects it to the wheels, which is a fourth arrangement again.

Parts: what simply is not there

An electric drivetrain has no fuel tank, fuel pump, injectors, exhaust, catalytic converter, particulate filter, turbocharger, multi-speed gearbox, clutch, or oil and filter service. Diesel adds the most of any of the three, because modern emissions limits require particulate filtration and usually a urea dosing system on top.

This is the commercially interesting difference, and the reason it is worth a paragraph on a markets site. Those absent subsystems are supplier revenue, assembly labour and scheduled servicing, which is why the transition is contested by more of the industry than the makers alone.

Sources

1. US Department of Energy, All-Electric Vehicles: where the energy goes, read 17 September 2026
2. US Department of Energy, Gasoline Vehicles: where the energy goes, read 17 September 2026