If your car original fuel sensor is analog, You could connect it to Teltonika device AIN input. But after that, You will need to do a calibration of the AIN values, that Your server could interpret AIN values. For example, fill the fuel tank fully and see what AIN value is sent to server. Than You will need to pour off for example 5 liters of fuel and check AIN value again. Continue to do that till the fuel tank is empty. After that You will have a table with one column of AIN values and other column of fuel level. So with that You could implement into your server AIN values which stands for exactly amount of fuel.
I hope I written the answer clearly.