Mileage isn't hard to figure. You keep your tach at a certain rpm, use your gps for land speed, look at the onboard computer for the consumption rate and you can get a mpg figure.
For example if at 4500 rpms you have a ground speed of 48 mph and your consumption rate is 6 gph then you'd be getting 8 mph.
You can't use the boat speedo because of too many wind and wave variables.