Fuel flow guage off 15%, suggestions???

I have a 2004 boat that I bought used this spring. It has the digital fuel management system with twin 150's. Filled up and zero'd out the system when I put the boat in the water. Just filled up for the first time and the fuel flow reading told me I needed 111 gal but the actual amount for the fill up was only 94 gal. This is 15-17% off and the system can only be calibrated 3%. Any suggestions on what to look at and how to remedy this would be greatly appreciated.

Thanks

[admin]

It can be set -4% if the gauge is reading too high but no more than that. Need to record fuel usage and gauge readings for three trips and then use totals to make corrections.