One thing that occurred to me about speeds reading through that old post (although I don't know how much difference it would make) are that there are two different ways that cycle computers can calculate average speed. One is auto stop/start (where it only counts actual riding time ie while the wheels are spinning - and divides the distance by that) where time stopped at lights etc doesn't count. The other computer setting is where you push 'start' at the start of your ride and 'stop' at the end so time stuck at lights counts - which would give you a lower average speed.