Hi
What Kieran wrote about two sensors is true.

> There are already approved fuel flow sensors available that are suitable
> for petrol. You might have a problem in that you probably need two sensors,
> one to measure fuel going to the carb. and another measuring the amount of
> unused fuel returning to the tank.
>
> R.S. in the UK sell fuel flow sensors last time I looked....They cost about
> 60 UK pounds each!
>
> Kieran

However, way back I used a cheap electrical petrol pump to measure
flow towards the carb. These pumps are actually solenoids attached to
a diaphraghm (OOOO! spelling) that suck a preset amount of fuel in
and then a spring push it out another way UNTILL the solenoid
triggers a hall effect device and it sucks again. So each stroke is a
fixed volume.  You might even end up using this pump AND a flow sensor in the
 return line. The
advantage of a pump is that it fills the float chamber before you
start the engine in situations where the car is not used for a long
period.

Stay well
Cassie