How to Measure Petrol Mileage with a Fuel Log
Use litres, distance and receipts to calculate km/l and cost per kilometre. Compare fuel grades without treating one trip as proof of a mileage improvement.
Key Takeaways for Indian Motorists:
- Use actual litres purchased and distance covered.
- Compare several similar journeys and record important differences.
- Calculate both km/l and rupees per kilometre.
Table of Contents
The two numbers to calculate
Mileage in km/l = distance travelled / litres used.
Fuel cost per km = price per litre / mileage in km/l.
These answer different questions. A fuel can produce a slightly higher distance per litre and still cost more per kilometre. Keep both figures in your log before comparing products.
Use consistent fills
Begin from a repeatable fill level using the normal automatic shut-off; avoid topping off. Record the odometer and retain the receipt. At the next comparable fill, record the distance and the litres needed to return to that level. This gives a practical estimate of consumption over the interval.
A single interval can be distorted by how the dispenser stops, the vehicle's position or the amount left in the tank. Several comparable intervals are more informative than one unusually good result.
A worked example
Suppose the distance between fills is 420 km and the replacement fill is 28 litres. The estimated mileage is 420 / 28 = 15 km/l. At an illustrative price of Rs. 105 per litre, the fuel cost is Rs. 7 per km.
These numbers are examples, not current market prices or measured results for a particular vehicle. Enter your own values in the fuel-cost calculator.
What should a useful log contain?
| Field | Why keep it? |
|---|---|
| Date and odometer | Defines the measurement interval |
| Litres and amount paid | Separates consumption from price |
| Product and outlet | Identifies the purchase |
| City or highway use | Gives context to the comparison |
| Load, air conditioning and traffic | Records differences between trips |
| Previous fuel in the tank | Identifies a mixed-fuel interval |
Keep any claimed ethanol percentage separate from the transaction data unless it was established for that fuel.
Can a mileage change prove ethanol content?
No. A consumption result is not a composition test. The DOE fuel-properties comparison shows that fuels can differ in energy per unit volume, but a road result also reflects the vehicle and journey. Do not convert a percentage change in km/l into an assumed ethanol percentage.
How do I compare several fills?
Add the total distance and divide it by the total litres over comparable intervals. This weights the result by the actual fuel used. Keep an unusual trip in the record and explain why it differs rather than silently selecting the best result. That makes your own conclusion easier to revisit.
Sources and Further Reading
Sources checked on September 7, 2026. Product information and local availability can change.
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