Bezier, I'm surprised as well, however, my actual speed is measured by my computer, not guesswork. Unless it became suddenly un-calibrated when I switched tires, same size mind you, it's an accurate estimate. I also judge speed using the clock: it's taking me longer to get to work/home, other riders, and perceived feel of input and output of energy. I'm definitly slower however, it's not entirely related to tire pressure. I know the other factors are: tire weight, compound, tread, punture resistant layers, etc. It's just that the tires are incredibly squishy and poor handling, the bike is slower rolling even when coasting downhill or under consistent pedalling on flat terrain, the hills are especially tough. I've used many types of tires, however, these have the lowest max. pressure of any I've used, thus my conclusion is that tire pressure is probably the biggest culprit in this case.