Gary,
You have probably already done it but the Northstar fuel meters should be calibrated after hook up. They claim 5% variance out of the box and can be tuned to 2%. Common way to dial it in is to run directly from known quantity like a portable 6 gallon tank. And then follow page 7 calibration instructions. Also Northstar suggests that the flow meter is positioned vertically, and that the gauge is at least 12" from compass and 20" from any radio.
You might also check to see if it was reading Imperial gallons rather than US gallons (also page 7). If that's the case you'd be getting 5.9 us gph for 4.5 mpg - still pretty darn good. If all is well then you're gonna be one happy fisherman at the fuel well
