Here is a web site that makes calculating TAS fairly simple. Using three GPS ground speed readings at 90 degrees to each other.
http://www.csgnetwork.com/tasgpscalc.html

Basically, climb to 7500, full throttle which is about 75% at that altitude, take a reading, turn 120 degrees, take a reading, turn 120 degrees and take a final reading. Average the three ground speeds and you got your number. This method takes winds aloft out of the equation.