I also imagine, ElectriCar that you 13 miles took you less than half the time than the 31 miles so your batteries have less time to let the chemistry catch up. So a 25% loss of apparent capacity would lead to a significantly greater loss of range if you drove at the same average speed. Lights and heat similarly compound the situation. So for me I'm not suprised by a 40% loss of range, it is not (according to my logic above which I hope is written understandably and is scientifically correct) indicative of a 40% los of initial capacity.
Is there, in fact, an actual loss of capacity in a cold battery, or just a slowing of the chemistry which lowers the one hour (C1) rate? (Or any other C arting, of course.)