Diesel or petrol: an honest comparison by cost of ownership, not advertising

The diesel-or-petrol argument is usually conducted around fuel consumption. That is the least important argument available, because it ignores the thing that matters: how you actually drive.

The main question: what is your route

Everything else follows from the answer.

If most of your journeys are under six to nine miles — a diesel does not suit you, however much it saves on fuel.

The reason is not the engine but the emissions systems:

• The particulate filter never completes a regeneration. Loading rises, aborted regenerations accumulate, and the filter dies within a few years. Mechanics in the DPF guide.
• The EGR valve clogs with soot faster — codes P0401, P0402.
• The engine never warms, condensation does not evaporate, the oil thins.

The cost of one particulate filter consumes several years of fuel savings in advance.

If you drive mostly on the highway and cover distance — a diesel operates in its designed regime and its advantages are real.

What costs more on a diesel

An honest list, because nobody mentions it at the point of sale.

The injection system. Injectors and the high-pressure pump are the most expensive consumable components. They are also sensitive to fuel quality and water; covered in common rail injector diagnostics.

The particulate filter.

The turbo. Petrol engines do not always have one; modern diesels always do.

Glow plugs — a seasonal consumable; covered in their own article.

A dual-mass flywheel on some models.

Servicing costs more overall: greater oil volume, more expensive filters.

What costs more on a petrol

For the comparison to be honest.

Fuel consumption — obviously, especially in town and with larger engines.

The catalyst — also not cheap, and on direct-injection engines it suffers from carbon.

Intake valve carbon on direct-injection engines is an expense category of its own; covered in the GDI carbon article.

The timing chain on some modern turbo engines is a topic costing more than many diesel items; covered in the P0016 article.

Simple arithmetic

You can do this on an envelope.

1. Annual mileage — the real figure.
2. The fuel price difference over that mileage for a year.
3. The purchase price difference.
4. Expected repairs over three years: for a diesel budget a particulate filter or injectors; for a petrol a catalyst or a chain, depending on engine.

If the diesel still wins after that, it wins. Under 9,000 miles a year it usually does not.

What to check when buying each type

Diesel:

• Particulate filter regeneration statistics — the number of aborted ones is most informative.
• Rail pressure: commanded versus actual, codes P0087, P0089.
• Boost under load, P0299.
• The EGR valve.
• Glow plugs.
• The water-in-fuel sensor.

Petrol:

• Per-cylinder misfire counters.
• Fuel trims across three operating states.
• P0420 and oxygen sensor condition.
• A cold start — on turbo engines, listen for the chain.
• Oil consumption on the dipstick.

The general protocol is in the used car inspection guide.

When the answer is obvious

Diesel, definitely, if: over 15,000 miles a year, mostly highway, frequent long journeys, or you need torque for a trailer or loads.

Petrol, definitely, if: under 9,000 miles, urban short trips, the car often stands, or the repair budget is limited.

A hybrid is worth considering if your route is urban and short — that is precisely the regime that kills a diesel and suits a hybrid best. Plug-in hybrid specifics are in their own article.

In short

• The route decides, not fuel consumption.
• Short urban trips kill a diesel through the particulate filter and EGR.
• One particulate filter consumes several years of fuel savings in advance.
• Petrol has its own expensive topics: catalyst, valve carbon, timing chain.
• Under 9,000 miles a year a diesel usually does not pay back.

Download the app — regeneration statistics, rail pressure, trims, and misfire counters are read with an ELM327 adapter over Bluetooth SPP on Android.