How do we section dynamic arrays for running averages, running slopes or other functions?

isaacmooreuky 

I have attached a couple of variations that may be of interest to you.

= LET(
  SpeedEcon, FILTER(Economy#,Speed#=@DistinctSpeed#),
  mean,      AVERAGE(SpeedEcon),
  median,    MEDIAN(SpeedEcon),
  stddev,    STDEV.S(SpeedEcon),
  count,     COUNT(SpeedEcon),
  CHOOSE({1,2,3,4}, mean, median, stddev, count) )

The first (green) performs the computationally intensive FILTER operation only once for each speed and outputs multiple statistical measures as an array.

The second (orange)  is closer to providing the spilt ranges you were hoping to achieve, but median and standard deviation might require helper ranges.

= AVERAGEIFS(Economy#, Speed#, DistinctSpeed#)

= COUNTIFS(Speed#, DistinctSpeed#)