The Gpd will vary depending on the RO membrane you use as each as a specific rating that should not be exceeded. That is why a flow restrictor is used. Another factor that can slow down a unit is the incoming water pressure. Ideally, the water pressure should be at least 40psi, but I have read other places where people recommend 45 or 50. My water pressure is really good so I run my unit at 50psi. I have a 75 gpd RO/DI unit and it makes ~4 gal/hr.

Ray, I'm curious where you bought your filters and membranes? I think you payed way more than you should have. I just replaced all 3 filters and my RO membrane on my unit and with shipping it only cost me $60. Got them from TBaquatics. Just thought you would like to know.