Frequently Asked Questions
Is my data private?
Yes. All calculations happen entirely in your browser. We don't send your inputs to any server, don't store your data externally, and don't require an account. Your saved inputs are stored in your browser's localStorage on your device only.
Is this professional financial advice?
No. This calculator provides estimates for informational purposes only. We are not financial advisors and do not provide personalised advice. Always consult a qualified professional for financial decisions.
How accurate are the results?
Our calculations use standard fuel consumption formulas and are mathematically accurate based on the inputs you provide. However, real-world fuel costs vary depending on driving conditions, traffic, weather, vehicle condition, and driving style. Treat results as estimates, not guarantees.
When was the calculator last updated?
The calculator and fuel price defaults are reviewed and updated regularly. The current default fuel prices reflect UK averages as of March 2025. You can always enter your own current fuel price for the most accurate results.
Can I export my results?
Yes. After calculating your results, an Export PDF button appears. This generates a clean, print-friendly PDF with your inputs, full results breakdown, and a timestamp — suitable for expense reports or budget planning.
How do I report an error or bug?
Please use our contact form to report any issues. Select 'Report a bug or error' as the subject and describe the problem. We review all reports and aim to fix confirmed issues promptly.
What's the difference between MPG and L/100km?
MPG (miles per gallon) is the UK/US standard — higher numbers mean better efficiency. L/100km (litres per 100 kilometres) is the European standard — lower numbers mean better efficiency. Our calculator supports both. See our L/100km to MPG guide for detailed conversions.
Why is my real-world MPG lower than the manufacturer's figure?
Manufacturer MPG figures come from controlled laboratory tests (WLTP). Real-world driving includes variables like traffic, hills, wind, temperature, and driving style that typically reduce MPG by 10-20%. We recommend calculating your actual MPG using the brim-to-brim method described in our MPG guide.
Does the calculator work for electric vehicles?
We include an electric option in the fuel type selector, but electric vehicle costs work differently (pence per kWh rather than pence per litre). The calculator provides a rough estimate for EVs, but a dedicated EV cost calculator would give more precise results for electricity costs.
Can I use this for business mileage claims?
The calculator shows your actual fuel costs, which is useful for comparison. However, HMRC has specific approved mileage rates (45p per mile for the first 10,000 miles, 25p thereafter) that may be used instead of actual costs for tax purposes. Check current HMRC guidance for your situation.
Why do petrol and diesel prices differ?
Diesel costs more per litre due to higher refining costs and different tax treatment. However, diesel engines are more fuel-efficient, so the cost per mile is often lower despite the higher per-litre price. Use our comparison mode to check which works out cheaper for your journey.
How often should I check my car's real MPG?
We recommend tracking your MPG over 3-4 fill-ups when you first start, then checking quarterly. Significant drops in MPG can indicate maintenance issues like under-inflated tyres, dirty air filters, or engine problems worth investigating.
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