Like has been stated many times, speed day to day is affected by too many things, so I don't really compare from one day to another. However, I do my intervals on a loop, and without a power meter, I still like to know that I am putting out basically the same effort from interval to interval, rather than fading into oblivion. So, I do use my computer to compare these efforts. If I can't keep my speed up, my workout is over.