Why fuel data is hard to read
A truck fuel gauge is not a laboratory instrument. On the road the fuel moves inside the tank, so the reading rises and falls with every bend, hill and braking. A naive system that alerts on any fall of the number will flood the office with false alarms, and after a few weeks nobody looks at them.
Many trucks also have several tanks connected to each other. After a fill the tanks do not settle at once: one reading can be high while another is still catching up, and the total keeps changing for a while. Reading the level right after a refuel gives a distorted picture.
The practical consequence: on the road a falling gauge is normally just consumption, and a rising one is often sloshing. Real theft happens differently, because fuel is taken from a parked vehicle. That is where you need to look.
Where the fuel data comes from
On most modern trucks the fuel level, the total fuel used and the current fuel rate are published on the CAN bus. A GPS tracker connected to it (the Teltonika truck trackers read this through CAN/FMS) can pass these values on without any sensor in the tank.
Before you buy anything, check that your truck model really publishes fuel on CAN. Some vehicles do not, and then the numbers are simply not there, whatever the tracker can do. Checking the model first saves a wasted installation.
Where CAN carries no fuel data, separate fuel-level probes installed in the tank exist on the market. They are a separate project with their own installation and calibration, so ask what your tracker and platform actually support before choosing one.
How a sound detection works
A reliable method rests on three ideas, and together they remove most false alarms.
- Evaluate only across a stop. Compare the fuel level before the vehicle stopped with the level after it. While driving, a falling gauge is consumption and is ignored.
- Use medians, not single readings. Take the median of the readings over several minutes before and after the stop, so that sloshing cancels out.
- Cross-check with the engine. Compare the drop with the engine's own fuel-used counter or fuel rate for the same period. If the engine burned that fuel (for example during long idling), it is not theft. If the tank lost more than the engine used, it is worth a look.
Alerts that actually help
An alert should raise a specific question, not just make noise. A short set of rules is enough for most fleets:
- Fuel drop with a litres threshold. Set it above the normal measurement noise of your vehicles and lower it later if you want more sensitivity.
- Ignition on or long idling at night, when nobody should be working.
- Leaving a depot geofence outside working hours.
- Unexpected stops away from planned points, especially long ones.
- Night parking outside the depot geofence.
An office routine that works
Technology only produces leads. What reduces losses is a simple, regular routine in the office:
- Review all fuel drops once a week, with place and time, and mark each as explained or not.
- Compare refuel events with the fuel-card statements: litres, time and place should agree.
- Compare consumption per 100 km between similar vehicles on similar routes.
- Talk to drivers with facts (place, time, litres), not suspicions. Many differences are innocent: a faulty sender, a leak, a wrong fuel-card entry. Check the vehicle before accusing anyone.
- Use eco-driving scores to separate fuel wasted by driving style from fuel that is really missing.
Signal, meaning and what to check
Use this table as a first triage when an alert or a strange number appears.
| Signal | What it can mean | What to check |
|---|---|---|
| Gauge falls while driving | Normal consumption | Nothing, unless consumption per 100 km is unusually high |
| Gauge jumps up and down on the road | Fuel sloshing in the tank | Ignore single readings; compare medians across a stop |
| Level lower after a stop, engine's fuel counter shows little use | Fuel taken from the tank, or a leak | Place and time on the map, signs of a leak, who had the vehicle |
| Level lower after a stop, engine's fuel counter shows matching use | Fuel burned by the engine, for example idling | Idling time and whether it was needed |
| Refuel on the map does not match the fuel-card statement | Fill for another vehicle, or a wrong entry | Receipt, card statement, litres and place |
| Ignition on at night on a parking | Unauthorised use or long idling | Alert history, who was assigned |
| High consumption per 100 km, no drops | Driving style, load, or a technical fault | Eco-driving score, load, service history |
| No fuel values at all | Truck does not publish fuel on CAN, or a connection problem | Vehicle model and tracker connection |
How Tafi GPS helps
Tafi GPS reads fuel level, total fuel used and fuel rate from the CAN bus through the tracker, so on most modern trucks no extra sensor is needed. Because some vehicles do not publish fuel on CAN, Tafi checks the models before devices are ordered. Consumption is shown per trip, per 100 km and per period, and refuels are detected automatically with time and place. Details are on the fuel monitoring page.
Fuel drops are evaluated only across a stop: the level before and after is compared using medians over several minutes, and the drop is cross-checked against the engine's fuel-used counter or fuel rate, so fuel burned by the engine is not reported as theft. A «fuel drop» alert rule with a litres threshold notifies the office, and drop and refuel events appear on the map where they happened. Ignition, idling and geofence alerts (unexpected stops, night parking outside a depot) and eco-driving scores complete the picture; see GPS for trucks.
Try it on a sample fleet with the free 7-day demo, or contact us to check whether your truck models publish fuel on CAN.