Calculate flow rates and velocity in pipes

Greetings all. This is a real world issue that is happening at work and there is much discussion as to the correct answer. The situation is this:

We need to test a Flowmeter that measures flow and velocity. Flow is in Gal/Min and Velocity is in Ft/sec.

There is an Omega Flowmeter installed in the pipe as our standard reference. Our product utilizes two (2) Transducers mounted on opposite sides of the pipe to measure the flow. They should read approximately the same amounts. The water is in a closed loop system driven by a centrifugal pump and controlled by a VFD Motor Controller. Both the reference meter and the transducers are mounted on an 8-foot horizontal pipe. The reference uses internal magnetic sensors to measure the flow.

The problem is that our product reads approximately 20 percent lower than the reference meter. One camp claims it is because there is a slight difference in the internal diameters of the pipe and the other believes it is noise generated by the Motor controller. Here are the particulars. Assume these numbers are EXACT (which they are).

Internal diameter of Omega Reference Meter is 1.87 inches.
Internal diameter of piping is 2.028
The reference reads 20 gal/min
Our flowmeter reads 16 gal/min.

Unfortunately, the reference meter does NOT display Velocity, only flow, so the velocity can only be calculated.

I have done numerous calculations based on info found online, but  am still stuck.

What say you all?

Thanks