I was wondering, I have managed to get my plane flying at about 550 knots (747-400) with a full tank(s) of fuel and just wanted to know how fast I am going in mph groundspeed?? I am at 40,000 feet with IAS at about 260knts i think
1 knot = 1.15078030303 mph
You asked what your ground speed was. To know that, you also have to know wind direction and velocity. 500 knots TAS with zero wind will give you ground speed of 500 knots. With a 100 knots of wind on the nose, grounds speed will be 400 knots. With a direct tailwind of 100 knots, ground speed would be 600 knots.
In real life, you can ask ATC your groundspeed or, if equipped, get it from your GPS or DME. You can also calculate it by measuring time to fly a known distance.
Just to add to the above GS display on the DME is only accurate when flying directly to/from the equipment if it is at a tangent you do not get a true reading.