Maximum range/best economy is not a simple calculation. First you need to know the exact fuel consumption (litres per hour) of the aircraft, at the weight that you are flying at, at a range of AIRSPEEDS (AS). Then you need to know the GROUNDSPEED (Gs) you are achieving. If you know the litres per hour (As) and divide by the miles per hour (Gs) you will get a figure for the litres per mile. Then consider if you fly a few miles per hour faster (As), how does that change the litres per hour and how does it change the groundspeed (Gs)? Run the calculation again. Find the speed that give the minimum litres per mile. Change the miles to km or nautical miles and litres to gallons or US gallons or pounds to suit your needs.
The microlight competition types go out and fly, to calibrate their fuel usage at precisely 40mph, 45mph, 50mph, 55mph etc and measure the exact quantity of fuel used at each speed. This gives them the fuel consumption to speed relationship for their aircraft which they use later in competition.
What you will find is that it pays to fly faster than the normal best economy speed if flying into a headwind and slower if flying with a tailwind.
There are a few fuel computers about that measure the fuel flow with an impeller and take a groundspeed reading (from a GPS) and do the calculation in real time.
If you are after maximum duration this is much simpler. It is simply the AIRSPEED that gives the lowest litres per hour.
Rans6...