I roll faster on the cooler weather commutes most of the time since I mount up the less trail spec tires in the off season. Cold temps usually drive me to stay warmer too.
On the aero equipped mtb I do about 16-18 on the flats/backroads and 27 in the city. Trail version about 14-17 flats/backroads and 25-27 city. In the city the limitation usually falls on the aero one due to the bar-end shifters and less powerful mechanical brakes. The trail mtb supports more aggressive city play at the expense of a little top end gearing, though in the winter I find it a little more comfortable in gloves.