The gas tank gauge on my 2170 is just about useless. I have the Yamaha digital gauges and the fuel level just moves 1 bar off of full at 21 gallons used in the 50 gallon tank. Thats what I'm talking about, useless. Theres not much to be done about that though, the tank is shallow and long making accurate level readings near impossible in a pitching boat.
I have a Navman 2100 fuel meter on my boat that shows the GPH and total fuel used. The Navman is my fuel gauge. With the Navman and the GPS speed I can read my MPG at anytime!
So 64 miles/21gallons, you at least got something above 3 mpg.
Blazer Bay 2170, Yamaha F150, Minkota autopilot, Garman 178c