Fuel Cost Calculator

Fuel is consistently one of the largest ongoing costs of vehicle ownership — second only to depreciation for most drivers. Whether you are budgeting for a long road trip, comparing running costs between vehicles, or trying to understand your annual petrol spend in the UK or gas spend in the US, this comprehensive guide covers every aspect of fuel cost calculation, from the basic formula to hypermiling strategies and EV comparisons.

The Fuel Cost Formula

The fundamental fuel cost calculation is straightforward:

Fuel Cost = (Distance / MPG) x Price per Gallon

For example, a 300-mile journey in a car achieving 35 MPG with petrol at $3.50/gallon costs: (300 / 35) x $3.50 = $30.00. In the UK with diesel at 155p/litre and fuel economy of 55 MPG (imperial): (300 / 55) x 6.78 litres/imperial gallon x £1.55/litre = approximately £57.33.

The key complication — and source of enormous confusion for US/UK comparisons — is that the US gallon and the UK imperial gallon are different sizes.

US Gallon vs UK Imperial Gallon

This is critical: if a UK car is rated at 60 MPG and an equivalent US car at 50 MPG, they are actually almost identical — the UK car achieves only 50 US MPG (60 x 0.8327). Never directly compare UK and US MPG figures without conversion.

In Europe (including many UK comparisons post-2018), fuel economy is also expressed as litres per 100 km (L/100km): L/100km = 282.5 / UK MPG, or L/100km = 235.2 / US MPG.

Current Fuel Prices: UK and US

UK Petrol and Diesel Prices

UK fuel is sold by the litre. As of 2024, average UK pump prices are approximately 145–155p/litre for unleaded petrol (E10) and 150–160p/litre for diesel. Prices vary significantly by region (motorway services charge a premium of 15–25p/litre over supermarket forecourts) and by fuel grade (premium 97/99 RON petrol costs 10–20p/litre more than standard E10).

UK fuel price breakdown: approximately 52.95p/litre fuel duty (frozen since 2011, extended through 2024 by Chancellor), plus 20% VAT on the total price, plus the wholesale fuel cost and retailer margin. Fuel duty alone accounts for roughly one-third of the pump price.

US Gas Prices by Region

US fuel is sold by the US gallon. Regular unleaded (87 octane) prices in 2024 range from around $3.00/gallon in Gulf Coast states (Texas, Louisiana) to $4.50–$5.50/gallon in California. The EIA (US Energy Information Administration) publishes weekly average gas prices by state at eia.gov. Key price factors include federal fuel tax (18.4 cents/gallon), state taxes (ranging from 8.95 cents in Alaska to 77.9 cents in Pennsylvania), refinery capacity, and proximity to supply infrastructure.

Annual Fuel Cost Calculation

To calculate your annual fuel spend:

1. Determine your annual mileage (UK average: approximately 7,400 miles; US average: approximately 14,500 miles)
2. Find your vehicle's real-world MPG (use the fill-up method rather than manufacturer figures)
3. Use current local fuel prices
4. Formula: Annual Cost = (Annual Miles / MPG) x Price per Gallon

Example UK calculation: 7,400 miles / 45 MPG (imperial) x 6.78 litres/gallon x £1.50/litre = £1,680/year. Example US calculation: 14,500 miles / 28 MPG x $3.40/gallon = $1,761/year.

Hybrid, EV, and Petrol Running Cost Comparison

Home EV charging is significantly cheaper than public rapid charging. In the UK, smart off-peak home charging (overnight) at rates such as the Octopus Go tariff (around 7–10p/kWh) dramatically reduces per-mile costs compared to 70–80p/kWh at motorway rapid chargers.

Hypermiling Tips to Reduce Fuel Costs

Hypermiling refers to driving techniques designed to maximise fuel economy beyond standard expectations. Key techniques include:

• Maintain correct tyre pressure: Under-inflated tyres increase rolling resistance. Under-inflation by 10 PSI can cut fuel economy by 1–2%.
• Smooth acceleration and braking: Anticipate traffic flow to avoid hard acceleration and heavy braking. This is the single biggest fuel-saving habit.
• Optimal speed: Most petrol cars are most efficient at 45–55 mph. Driving at 70 mph instead of 60 mph increases fuel consumption by around 25%.
• Reduce air conditioning use: AC can increase fuel consumption by 5–15% at city speeds.
• Remove roof racks and boxes: A roof box at motorway speed increases drag and can cut fuel economy by 15–25%.
• Reduce vehicle load: Every extra 100 lbs (45 kg) of weight reduces fuel economy by approximately 1–2%.
• Engine warm-up: Modern cars do not need extended idling warm-up. Drive immediately but gently until the engine reaches operating temperature.

Fuel Economy Standards: US CAFE and UK/EU CO2

In the United States, the Corporate Average Fuel Economy (CAFE) standards require new passenger cars to average approximately 49 MPG (combined) by 2026 under EPA rules. Light trucks and SUVs have separate, lower standards. Manufacturers that fail to meet CAFE face fines of $5.50 per 0.1 MPG per vehicle sold below standard.

In the UK and EU, regulations are expressed in grams of CO2 per kilometre rather than MPG. The EU target for new passenger cars is 95 g/km CO2 (fleet average). Post-Brexit, the UK maintained similar standards with 95 g/km targets. For petrol cars, approximately 95 g/km CO2 equates to around 49 MPG (imperial) / 41 MPG (US).

Electric Vehicle MPGe

The US EPA rates electric vehicles using MPGe (miles per gallon equivalent), calculated by determining how far an EV travels on the energy equivalent of one US gallon of gasoline (33.7 kWh). A Tesla Model 3 achieves around 132 MPGe combined. This allows direct comparison of EV and petrol vehicle efficiency.

The UK uses a similar concept but often expresses EV efficiency as miles per kWh (mi/kWh) — a figure of 3.5–4.5 mi/kWh is typical for modern EVs in real-world conditions.