Diesel vs Petrol Running Costs: Which Is Cheaper?

Key differences between petrol and diesel engines

Petrol and diesel engines both use internal combustion, but they work in different ways.

Petrol engines use spark plugs to ignite a fuel–air mixture. They are usually:

• Quieter and smoother
• Lighter and simpler
• Better suited to short trips and stop–start driving

Diesel engines compress air and inject fuel directly into the cylinder. The heat from compression ignites the fuel. They are usually:

• More fuel-efficient
• Better at producing torque for motorway driving
• More complex due to emissions systems

These mechanical differences influence running costs over time.

Purchase price comparison

Diesel cars are often more expensive to buy than their petrol equivalents.

Typical reasons include:

• Stronger engine components
• Higher-pressure fuel systems
• Additional emissions equipment such as DPFs and AdBlue systems

In many cases:

• Diesel versions of the same model cost £1,000–£2,500 more than petrol versions when new.
• On the used market, the difference depends on age, mileage, and demand.

If you do not drive enough miles to benefit from diesel fuel savings, this higher purchase price can outweigh any fuel economy advantage.

Fuel economy differences

Diesel engines are generally more efficient than petrol engines.

Typical patterns:

• Small petrol hatchback: 40–50 mpg
• Small diesel hatchback: 55–70 mpg
• Medium petrol family car: 35–45 mpg
• Medium diesel family car: 50–60 mpg

Diesel engines are most efficient on:

• Long motorway journeys
• Steady speeds
• High annual mileage

Petrol engines are often more efficient in:

• Short urban trips
• Stop–start traffic
• Cold-weather short journeys

Fuel price comparison

Diesel fuel is usually slightly more expensive per litre than petrol in the UK, though the gap changes over time.

Typical pattern:

• Petrol: slightly cheaper per litre
• Diesel: slightly more expensive per litre

However, because diesel engines often use less fuel per mile, they can still be cheaper overall for some drivers.

Cost per mile comparison

Cost per mile depends on:

• Fuel price per litre
• Real-world mpg

You can estimate:

• Cost per mile (£) = fuel price per litre ÷ miles per litre

Example scenario:

• Petrol at £1.45 per litre, 45 mpg
• Diesel at £1.55 per litre, 60 mpg

Even though diesel costs more per litre, the higher mpg can result in a lower cost per mile.

This is why diesel is often favoured by:

• High-mileage drivers
• Motorway commuters
• Taxi and fleet users

Road tax differences

Road tax (Vehicle Excise Duty) depends mainly on:

• CO₂ emissions
• Vehicle list price when new
• First registration date

In general:

• Older diesel cars without modern emissions systems may have higher tax.
• Modern diesels that meet current emissions standards can have similar tax to petrol cars.

For cars registered after April 2017:

• Most petrol and diesel cars pay the same standard rate after the first year.
• Expensive cars may also pay the additional rate.

Always check the exact tax band for a specific registration number.

Insurance cost differences

Insurance premiums depend more on:

• Driver age and history
• Car insurance group
• Postcode
• Annual mileage

However, there are some trends:

• Diesel cars can be slightly more expensive to insure if they are higher-value or higher-torque models.
• Petrol cars are often cheaper to insure in lower insurance groups.

The difference is usually smaller than the gap in fuel or maintenance costs.

Maintenance differences

Maintenance is where the biggest long-term cost differences often appear.

DPF systems in diesels

Most modern diesel cars have a Diesel Particulate Filter (DPF).

The DPF:

• Traps soot from the exhaust
• Burns it off during long, hot runs (called regeneration)

Problems can occur if:

• The car is used mainly for short trips
• The engine never reaches full temperature
• Regeneration cycles are interrupted

DPF faults can lead to:

• Warning lights
• Reduced performance
• Expensive repairs or replacements

Turbo and injector costs

Diesel engines rely on:

• High-pressure injectors
• Turbochargers

These components are:

• Precise and expensive
• Sensitive to poor maintenance or low-quality fuel

Repairs can cost significantly more than similar petrol components.

Petrol engine servicing

Petrol engines are usually:

• Simpler
• Cheaper to service
• Less dependent on complex emissions equipment

Typical petrol maintenance includes:

• Spark plugs
• Ignition coils
• Air and fuel filters
• Oil and filter changes

While petrol engines can still have expensive faults, they often have fewer high-cost emissions components than diesels.

Reliability considerations

Reliability depends more on the specific model than the fuel type, but there are some general patterns.

Diesel reliability risks:

• DPF blockages on short-trip driving
• Injector or fuel pump wear
• Turbocharger failures at high mileage

Petrol reliability risks:

• Ignition coil or spark plug faults
• Timing chain or belt wear (model-dependent)
• Higher fuel consumption over time

Driving style and maintenance history are often more important than fuel type alone.

Emission zone impacts (ULEZ and CAZ)

Many UK cities now operate:

• ULEZ (Ultra Low Emission Zones)
• CAZ (Clean Air Zones)

Older diesel cars are more likely to face charges.

General rules:

• Petrol: usually compliant from Euro 4 (around 2006 onwards)
• Diesel: usually compliant from Euro 6 (around 2015 onwards)

If you regularly drive into a zone, daily charges can dramatically increase running costs for non-compliant diesels.

Resale value comparison

Diesel cars have historically held strong resale values due to:

• Better fuel economy
• Demand from high-mileage drivers

However, trends have shifted due to:

• Emission zones
• Changing buyer preferences
• Growth of hybrid and electric vehicles

Today:

• Modern, compliant diesels still hold value well for long-distance drivers.
• Older diesels may be harder to sell in cities with emission charges.

Which is cheaper for different driving styles

City driving

Petrol is usually cheaper for:

• Short daily trips
• Stop–start traffic
• Low annual mileage

Reasons:

• Lower purchase price
• Fewer DPF-related issues
• Lower maintenance risk

Motorway driving

Diesel is often cheaper for:

• Long daily commutes
• Frequent motorway journeys
• Regular long-distance travel

Reasons:

• Better fuel economy at steady speeds
• DPF systems stay healthier on long runs

High-mileage drivers

Diesel usually works out cheaper if you:

• Drive 12,000–15,000 miles per year or more
• Spend a lot of time on motorways
• Keep the car for several years

The fuel savings can offset:

• Higher purchase price
• Higher maintenance costs

Final cost comparison scenarios

Low-mileage city driver

• 6,000 miles per year
• Mostly short trips
• Urban driving

Likely outcome:

• Petrol cheaper overall due to lower purchase and maintenance costs.

Average mixed-use driver

• 10,000 miles per year
• Mix of town and motorway

Likely outcome:

• Costs can be similar.
• Choice depends on specific model, mpg, and maintenance history.

High-mileage motorway driver

• 18,000 miles per year
• Mostly steady motorway driving

Likely outcome:

• Diesel usually cheaper due to significantly lower fuel cost per mile.

Quick checklist before choosing petrol or diesel

• I know my realistic annual mileage.
• I understand how much of my driving is city vs motorway.
• I have compared fuel economy figures for both versions of the same model.
• I have checked road tax and insurance for each option.
• I understand the maintenance risks of diesel DPF and injector systems.
• I have checked whether the car is compliant with local emission zones.