Flow sensor integration with the Car electronics system

You can obtain a simple Hall effect sensor tachometer display anywhere. If you need the display to read out in liters per minute instead of pulses per minute, you will need a smarter display.

Probably best to build your own using an Arduino, pull-up resistor for the sensor line, compatible display (LCD/OLED) and by writing your own code. I would suggest having pulses being counted by an interrupt routine and converted to a rate and total with some simple math (depending on your particular unit's specifications).

wouldn't using an arduino complicate things & make the device costly?  Sorry didn't mentioned before, looking to build a compact cost effcient device 

I suggest you do some research, especially if you think Arduinos and DIY is expensive.

How cheap do you think you'll be able to find an off-the-shelf product that meets all your needs? I work with these flow sensors - last time I looked, even basic totaliser units with a display that show quantity and rate for these units were US$200+, which is why I did exactly what I suggested as I was doing a project for developing-world applications.

- Gough

On some 'return flow' injection systems, you may need a second sensor so as you can subtract the return flow in order to get the actual fuel being used by the engine.

This device is to address short-fueling.(Yes, short-fuelling is a problem in many countries).
Not connecting with fuel pipe but fit around the tank's fuel cap so that when the vehicle is refueled, it notifies the actual volume filled in the vehicle.

Well, that's probably not an ideal sensor for this application. They're often no more accurate than +/- 10% depending on the flow rate.

- Gough

A flow sensor capable of giving accurate measurements of the amounf of fuel pumped into a motor vehicle during re-fueling would have to be the same bore as the existing fuel filler and installed properly in line with it. This would have to meet stringent regulatory requirements for safetey (especially in petrol powered vehicles). seperate approval would be needed for every vehicle type.

I don't know of an off the shelf flow sensor that would work as the core of the design.

I can't see the cost being less than a few hundred pounds/dollars per vehicle even if the product were made in reasonable numbers.

MK

Given that car dashboards can vary quite a bit, then you may find it easier to start off with a separate display.

There was an example of using a similar water flow sensor with an Arduino here:
https://wiki.seeedstudio.com/G1_Water_Flow_Sensor

which looks like it was based on an earlier fan speed display project:
https://www.themakersworkbench.com/content/tutorial/reading-pc-fan-rpm-arduino

which uses a similar hall effect sensor pulse output. It would then be pretty easy to display the result on a 2-line LCD display.

What sort of accuracy do you need to make this all worthwhile though ?

Given the limited accuracy of the device you have chosen, you might find that you can achieve more accurate results by using the fuel gauge already in most cars. Combine its output with a accelerometer data to measure the current angle of the fuel tank when measuring, and then output it on a display. 

On its own, the fuel tank is a pretty accurate measuring device and comes free with most cars. If you fill it from empty to full then you know how much fuel has been pumped into the tank. If the tank is 50L capacity and you are being charged for 60L then you know you are being scammed for an additional 10L that you didn't receive. Given the limited accuracy of the water flow meter, you wouldn't need to refill from empty to achieve the same result, just within around 10% (~5L) or so of empty, so perhaps 'low fuel warning light' is near enough.

Another pretty accurate measuring device that comes free with the car is the fuel injector. The ECU knows how much fuel it is delivering to the engine and thus how much is being removed from the tank. However its implementation is not standard across all cars.