Analog or Digital fuel sensor?

Digital (RS232, RS485, CAN) is better because it is "loseless". Tracker receives either correct message from sensor or does not receive anything.
Analog signal can be affected by bad wiring, bad "ground" contact. Also resolution of sensor in most cases is limited by ADC properties of GPS tracker.
So we highly recommend to use digital interfaces for fuel monitoring.

RS232, RS485 and CAN  will provide same precision, only difference that RS232 allows only one device to be connected while RS485 and CAN allows to connect several sensors.

it is a normal, more than 50% FLS users do not understand  accuracy correctly.

Accuracy of level sensor - it is accuracy of level measurement, not volume. If sensor has 1% accuracy that means that it measures fuel level with 1% error in static condition (not moving) and with heating and cooling of sensor during all temperature range.

(It does not count temperature expansion of fuel itself because level changes due to temperature expansion of fuel depends from the tank shape. But temperature expansion is almost compensated by dielectric constant changes  of fuel itself during cooling/heating. So, good fuel level sensor must have stable properties during cooling/heating and it  solves the problem of temperature.)

But customers always interested in volume measurement, right?
This accuracy depends from several factors:
1. Sensor accuracy (see above)
2. Tank calibration accuracy (depends from installer)
3. Tank shape (wide and low-height tank will generate more error than vertical tank, irregular shape will generate more error than regular)
4. Vehicle operation (uphill/downhill, in combination with irregular tank)

We usually talk to our customers these info:
In the Wialon you will see the current volume in the tank with 1% error (99% accuracy) calculated from the full tank volume
for example, for 100 and 500 liters tank you will have +/-1 and +/-5 liters error.

How Wialon calculates filling and theft volume : as a difference from 2 fuel volumes.
that mean we have double error (from 2 volume measurements).  and error of filling/drain volumes will be +/-2 and +/-10 liters accordingly.
it is a worst case though(double error).

Note, that this error does not depends from the filling/drain volume. NO matter, filling was 100 or 10 liters, error is same.
This model is very close to reality.

About ADC error :
Lets say GPS tracking device has 10-bit ADC which gives us 1024 points of resolution
But this analog input have 0-30V range.
Typical fuel level sensor with voltage output has 0-10V range (or even 0-5V)
so it is use only 1/3 or 1/6 of ADC range, and same part of this resolution.
for 0-10V sensor we still have only 300 points of resolution which is worse than sensor can provide, so we limit the sensor precision.

You are right! But you are talking about length, it is useless for customer. They need volume. Volume error better calculate from full tank volume.

About drain detection by FLS. Wialon has much more possibilities to detect fuel drain than sensor - speed data, vehicle position, sensor data, time etc.
Sensor can't make drain detection better than software.
It is used for emergency notification. Because Wialon detects fuel theft with delay (3-10 minutes). It can be very long if fuel is stolen by another person (not driver). Information about fuel theft is useless because it is evidently that fuel was stolen (driver is a first person who will say about it). We need instant driver notification to catch the thief.
So our sensor has alarm output as a bonus which can be connected to the alarm input of GPS. (rs485/rs232  can't work as alarm). And gps tracker will activate voice call/sms during robbery.
but volume of robbery will not detected.

to calculate how bad is analog interface for fuel monitoring you need to have data:
1. ADC resolution  and input range of analog input of gps tracker
2. voltage range of fuel sensor after cutting (some can be recalibrated some not).

approx initial resolution of cutted sensor is 1000 points per length (in theory, you can measure 0.1mm level change in very best case, 0.1%)
than calculate of analog input resolution : 10 bit =1024, 12bit=4096, 8 bit=256
than calculate which part of analog input you will use :     sensor range/input range.    lets say 10volt/30 volt=0.3

and you will see the smallest level change you can feel by sensor:

0.3*1023=300 points per height, for 500mm sensor it is 1.6mm delta. You will not see fuel change of 1 mm height.

So we should be careful in choosing the GPS Tracking Device, right?
And the most important thing is to choose a Fuel Level Sensor as accurate as possible.
This SOJI Analog Fuel Leve Sensor can achieve 99.5% accuracy, a really good bang for the buck.