this formula ,from a friend of mine who has since passed many years ago will give you a truer figure for todays
average speed.i would suggest using figures for the same distance & surface.
you will need at least 3 figures to work with,5 or more figures will be better.
subtract the two highest figures and the two lowest figures,then average out the rest to get a single figure.
i use this all the time & get better handicapping tool as a result.
good luck.