Fred,
Big Lew hit a home run with the points he makes.
This Lowrance unit comes with the fuel flow sensor. I believe paddle wheel tranducers etc are for other things like speed, water temp and any other measurements that may be electronically probed.
http://www.turtlemarine.com/lowrancelmf200.html
Northstar/Navman also makes a nice kit for about $125
I just have their flow sensor which plugs into my Sounder or GPS (which does read out the mpg), but it sounds like you're looking for just the gauge and sensor. The problem with many of these is that you need to know your actual fuel amount and reset accordingly. They are plug and play. place the sensor inline between the fuel filter and the motor and then run the wire back to the gauge or electronic device.
Here's a link for Northstar F210
http://www.northstarnav.com/en/Produ...ent/F210-Fuel/