If you want to move in "jumps" of 15 rows at a time, then you can either subsample the output of those functions or use blockproc in the Image Processing Toolbox. Or you can use a simple for loop:
% Compute speed over "blocks" of 15 elements
speed = t.Speed; % Get speed values from your table.
index = 1;
for k = 1 : 15 : numel(speed)
meanSpeeds(index) = mean(speed(k : k + 14));
index = index + 1;
end
