My 2 cents about the speed option is from the saltchuck point of view...Although a GPS can indicate speed, the speed it provides is SOG, speed over gound. That is not the same as the speed you are actually travelling through the water though, unless there is zero wind drift or measurable current. Used together with the speed indicator on the FF, you can get an idea of what the current speed is and then use that info to determine your approximate speed through the water.

Doesn't mean that it's worth the extra money of course, but can be helpful when you want to be as close as possible to your preferred speed through the water.

What I have found is using my Interphase is that the FF speed indicator is actually pretty close to what the GPS says, plus or minus the current speed. Usally within 1 to 2 mph if I'm running on plane. Slower speeds are effected by a lot of factors including weeds, sticks, wind, placement on the hull and God knows what else.

After a while, you just know what the right speed is anyway. On my boat, it's about 800 rpm on the 100 HP Suzuki.