Hi - the speed rating is always calculated after the race has finished so is only available for historic runners. We don't explain the basis for calculation, it's optional information that you can use at your discretion. Generally if the race was poor (a much slower than standard time) a speed rating is not returned, or if there is no benchmark for the runners. An alternative of course is to create your own speed ratings. We'll be writing more about this in the blog at the start of the new year.