Tank-to-tank measurement is better than guessing
To estimate real consumption, fill the tank and reset the trip meter or record the odometer. Drive under your usual conditions, refill the tank, and record both liters added and distance traveled. If the tank returns to a similar level, the added fuel approximates what that distance consumed.
- Fill the tank and record the odometer.
- Drive a useful distance under representative conditions.
- Refill and record liters and distance.
- Divide distance by liters, or liters by distance multiplied by 100.
The two formulas and their units
For km/L, a higher number is better because the vehicle travels farther per liter. For L/100 km, a lower number is better because less fuel is needed for the same distance. Do not say consumption “increased” without identifying the unit.
Example: 560 km using 42 liters
- km/L = 560 ÷ 42 = approximately 13.33 km/L.
- L/100 km = 42 ÷ 560 × 100 = 7.5 L/100 km.
- At an illustrative fuel price of 2.30 per liter, trip cost = 42 × 2.30 = 96.60, and cost per km = 96.60 ÷ 560 ≈ 0.1725.
Convert between the units with L/100 km = 100 ÷ km/L, and vice versa, allowing for rounding.
A table to prevent reversed interpretations
| Measure | Formula | Better direction | Example |
|---|---|---|---|
| km/L | Distance ÷ liters | Higher is better | 13.33 |
| L/100 km | Liters ÷ distance × 100 | Lower is better | 7.5 |
| Cost/km | Liters × price ÷ distance | Lower costs less | 0.1725 |
Why results differ from the brochure or another trip
FuelEconomy.gov identifies driving behavior, speed, air conditioning, load, vehicle maintenance, road conditions, and weather among the factors affecting fuel economy. One trip is not a permanent specification. Repeat the measurement and record city versus highway conditions; do not compare congested winter driving with a clear highway trip as if conditions were identical.
Common mistakes
- Using tank capacity instead of fuel actually added.
- Measuring over a very short distance, where refill variation matters more.
- Forgetting to convert miles or gallons when units differ.
- Assuming km/L and L/100 km improve in the same numerical direction.
- Using an old or unverified fuel price.
Frequently asked questions
Do I need to empty the tank?
No. Refill measurement estimates fuel used without emptying the tank, which should not be done merely for this test.
Is the dashboard reading definitive?
It can be useful, but compare it with tank-to-tank measurements over several refills to assess any difference.
How many times should I repeat the test?
There is no magic count. Several refills under similar conditions produce an average less affected by one refill's variation.
Sources and references
- FuelEconomy.gov — Factors that affect fuel economy: driving, vehicle, and environmental factors. Accessed 2026-11-06.
- The equations are direct unit conversions. The example fuel price is illustrative, not a current price.