Note: costs mentioned below are given in US dollars as a general point of reference — actual prices vary by country, currency, and local market.
For a 12,000-mile year, a petrol car averaging 30 mpg costs roughly $1,400-$1,800 in fuel depending on local prices, a diesel equivalent at 38 mpg often costs somewhat less per mile despite diesel's usually higher price per gallon, and a CNG-converted vehicle can cost 30%-40% less per mile than petrol where CNG infrastructure and pricing make it viable.
At a glance
| Fuel type | Typical efficiency | Relative cost per mile |
|---|---|---|
| Petrol | 25-35 mpg (segment average) | Baseline |
| Diesel | 32-45 mpg (segment average) | Often 10%-20% lower per mile |
| CNG | Similar mpg-equivalent to petrol | Often 30%-40% lower per mile where available |
What this estimator does
The calculator on this page works out fuel cost from three figures: your annual distance, your vehicle's efficiency, and the price you actually pay per litre. It returns cost per year, per month and per mile, plus the volume of fuel you get through in a year. To compare petrol against diesel or CNG, run it once per fuel with that fuel's own efficiency and price and set the results side by side — the arithmetic is identical for each, and doing it separately keeps you honest about the fact that the same car does not achieve the same annual mileage. The goal is a fair, per-mile cost comparison rather than a comparison based on pump price alone, since efficiency differences between fuel types matter as much as the price per unit.
Why pump price alone is misleading
Diesel is often priced higher per gallon/litre than petrol in many markets, yet diesel engines are typically more fuel-efficient, so the cost per mile can still come out lower or roughly even. CNG is usually priced significantly lower per energy-equivalent unit than either liquid fuel, and CNG vehicles get roughly similar mileage to their petrol counterparts, which is why CNG can offer the largest per-mile savings of the three — where filling infrastructure supports it.
Worked example: 12,000 miles/year
| Fuel type | Efficiency | Est. annual fuel cost |
|---|---|---|
| Petrol | 30 mpg | ~$1,600 |
| Diesel | 38 mpg | ~$1,450 |
| CNG | ~28 mpg-equivalent | ~$1,000 |
What changes the comparison
- Local price gaps — petrol, diesel, and CNG prices move independently and the gap between them varies significantly by region and over time.
- Driving style and conditions — highway-heavy driving favors diesel's efficiency advantage more than stop-start city driving.
- Vehicle age and maintenance — a poorly maintained engine loses efficiency regardless of fuel type, eroding any fuel-type advantage.
- CNG infrastructure availability — CNG's price advantage only matters if refueling stations are reasonably accessible in your area; scarce infrastructure adds detours that offset savings.
- Vehicle purchase/conversion cost — CNG vehicles or aftermarket conversions carry their own upfront cost that should be weighed against fuel savings over time.
How to use the estimate wisely
- Enter your annual mileage (or a typical commute distance multiplied out).
- Enter your vehicle's rated fuel efficiency for each fuel type you're comparing.
- Enter current local prices per gallon/litre for petrol, diesel, or CNG.
- Compare the annual cost outputs, then weigh any purchase or conversion cost difference against the savings, similar to the EV vs petrol break-even approach for electric alternatives.
Common mistakes to avoid
- Comparing pump price per unit directly without adjusting for each fuel's different efficiency — this is the single most common error.
- Using manufacturer-rated mpg figures without adjusting for real-world driving, which typically runs 10%-15% below rated figures.
- Ignoring maintenance cost differences — diesel engines often have higher service costs, and CNG systems may need periodic tank inspection/recertification.
- Assuming CNG savings apply everywhere — filling station density varies enormously by region, and thin coverage undercuts the theoretical savings.
Diesel and CNG prices can swing independently of petrol prices due to different supply chains and taxation — re-check current local prices before relying on last year's numbers. Real-world fuel economy is usually lower than the official rated figure; use a conservative efficiency estimate for a realistic annual cost.
Frequently asked questions
Is diesel cheaper to run than petrol?
How much can CNG save compared to petrol?
Does real-world fuel economy match the official rated mpg?
Is it worth converting a petrol car to run on CNG?
Which fuel type is cheapest for high-mileage drivers?
Sources & further reading
- U.S. Energy Information Administration — Gasoline and Diesel Fuel Update
- U.S. DOE Alternative Fuels Data Center — Natural Gas
Figures, prices and policy details were current at the last-updated date above. Automotive pricing, incentives and regulations change frequently — verify time-sensitive details with the linked primary sources. Read our editorial policy and fact-checking standards.
