On 3/26/2018 at 3:03 PM, fppilot said:

The first step is to find, or develop, a burn profile at various altitudes.  I do that by attaining a normal cruise speed at a given altitude, then logging fuel burn for say 15 minutes. Multiple that X 4 for hourly burn, and put that and the associated TAS for that altitude into the profile. Then I change altitudes, take a few minutes to stabilize, then add that altitude to the profile.  I build the profile in 2,000 ft increments, then I enter the profile into an aircraft performance profile at Fltplan.com, which is where I do my flight planning. 

 

27 minutes ago, downscc said:

All aircraft have very predictable TAS based on weight, pressure altitude and SAT and this is where we start when planning for jets. In fact, the dispatchers have software that knows exactly each aircrafts performance based on past performance.  This and statistical analysis of a specific route allow dispatchers to use less reserves on routes flown repeatedly by the same aircraft.

We are in total agreement. And that was my recommendation to the OP.  Profile.  Know the performance difference for altitudes in the normal range of consideration and then the difference in wind factors and the answer is  likely obvious.  Mostly based on performance knowledge and experience.

7 hours ago, fppilot said:

You are taking the complex formula-driven route and backing into GS by basing it on other calculated speeds that are dependent on additional influences . Leave TAS out of the question/answer

Ground Speed is a factor of time and distance.  Nothing else need enter the equation.

This is of course correct. However, your question was about how ground speed is influenced by altitude. The answer to that is a function of TAS and wind. How can it be otherwise?

You can retrospectively calculate the groundspeed using time and distance, but in a planning situation you don't know the time because you haven't flown it yet! You do, however, know the wind and the true airspeed that you will get at a given altitude and power setting...

7 hours ago, skelsey said:

You can retrospectively calculate the groundspeed using time and distance, but in a planning situation you don't know the time because you haven't flown it yet! 

We are still in agreement.  You just defined "profile".  Once you have developed a profile you indeed have flown it already.  Then you just apply the profile and factor in  the wind.  Here is one of my profiles I use at FltPlan.com.