The flow rate of the pumps bears no relationship to the amount of fuel used, unless the flow is too small. This can be verified by monitoring pressure while driving the car under full load, as Donnie has suggested.
I would suggest a couple other techniques other than taping to the windshield. The nicest is to use a pressure transducer and your trusty VOM. The way I have done it for years is to make a ten foot fuel line out of the aramid plastic fuel line used for VW etc injection line. Then you can set the gauge in your lap or on the dash.
The reason for the screw type pumps is cost IMHO.
Bosch Master, ASE Master, L1
33 years MB technician