Once apon a time (late 80's?) certain models of Ford cars with carburetors had an external fuel flow sensor in line with carburetor. The sensor was used for a onboard gas mileage calculator...

I had a GF years ago who had a Ford Capri (I think it was a Capri) with the 2.6L V6 and it had the mileage calculator and fuel flow sensor.

Maybe you could still score one at the auto parts store or wrecking yard.