why do you want to know the greatest endurance speed? Surely the more useful thing to know is the best fuel economy speed which, as someone else has posted, requires you to know Groundspeed and fuel flow rate. Staying airborne for hours on end does not guarantee you have covered the biggest distance on the tank full of fuel. You need to fly faster than normal best economy airspeed if you have a headwind and slower if you have a tailwind. This is very noticeable at the normal cruise speed of my Rans, 65 to 70 MPH.
Andrew.