1. The four measurement methods at a glance
Full-tank method (the everyday gold standard): Fill the tank to the brim, note the mileage, drive normally, then fill up to the brim again – liters refueled divided by kilometers driven times 100 gives the real consumption. Important: always use the same filling station and pump auto-shutoff, and average across several tank fills.
On-board computer: Convenient, but to be treated with caution – displays frequently deviate from the real value by a few percent, usually in the vehicle's favor. Still usable for before-and-after comparisons on the same vehicle, because the error stays constant; for absolute values, use the full-tank method.
Telematics / CAN data: The best solution for fleets: automatic recording per vehicle and driver, including idle share and driving profile. This makes it possible to evaluate measures statistically cleanly across many vehicles.
Liters per operating hour (l/h): For tractors, construction machinery, boats and everything that works in hours instead of kilometers: divide the liters refueled by the difference on the operating-hours meter. The figure becomes meaningful when the work in the comparison period is similar.
2. Distorting factors: what skews comparisons
The most common sources of error in before-and-after comparisons:
- Season: winter operation typically costs 5–15 % more due to cold starts, winter diesel and heating – never compare summer weeks against winter weeks.
- Route & load: a different route, more payload, trailer operation – and the comparison is worthless. Choose usage profiles that are as similar as possible.
- Driver: different drivers differ by double-digit percentages. Either keep the driver constant or average over many trips.
- Too short a period: individual tank fills scatter widely. Rule of thumb: at least 3–5 tank fills or several weeks per comparison period.
- Expectation effect: anyone who has installed a saving measure unconsciously drives more economically. The only remedy: choose a long enough period and do not change your driving style.
3. Step by step: the before-and-after protocol
- Establish a baseline: drive normally for 4–6 weeks (or 3–5 tank fills) and document: date, liters, km or operating hours, type of use.
- Implement only one measure – anyone who changes tires, does training and installs a system at the same time cannot attribute anything afterwards.
- Drive the comparison period: same duration, conditions as similar as possible (season, routes, driver, load).
- Evaluate: compare the average consumption of both periods; flag individual outliers (holiday trip, extreme weather).
- Translate the result into money: savings in percent × annual consumption × fuel price = annual amount – the basis of every payback calculation.
4. How professional tests do it
Laboratory tests eliminate the distorting factors by running standardized cycles: in the laboratory test of the FET system, five WLTC cycles and constant-speed runs at 50, 80 and 130 km/h were driven with and without the system on the same vehicle – exactly reproducible conditions, documented in the test report. Field tests work with reference periods instead: in the documented Unimog field test, consumption per operating hour was measured over a winter period against the comparison period (result: around 10.9 % reduction). The two methods complement each other: the laboratory delivers precision, the field test real-world relevance.
Conclusion
A clean measurement costs nothing except a little discipline – and it is the only currency that counts when it comes to saving measures. Our own standard here: measure it. A provider who is convinced of their technology will always encourage you to take your own measurements.