Amsterdam - most miles per capita

Maybe we don't have stats about trip lengths, but we do have stats about modal share in Ams (the app is in Spanish, sorry):

http://modalkombat.demimismo.com/amsterdam-versus-madrid

As you see, they have a still high car (Coche) share (comparing with Madrid), but a rather low pedestrian (A pie) share... so I think that your theory about the length of their trips may be true: the bike in the Netherlands is mainly an alternative for walking and using the public transport system, they still depend on their cars to make longer trips.

Here you can see a comparisson of two cities with similar sizes, again one from Spain (Oviedo) and another from the Netherlands (Eindhoven):

http://modalkombat.demimismo.com/oviedo-versus-eindhoven

Same trend!

If I could draw people's attention to this post, which makes a couple of good points.

The Netherlands have been somewhat less unsuccessful than the UK in resisting the advance of motor traffic, but the modal share of motor traffic over there is still well over 40% (60% in the UK, so, yes, we're doing far worse on that score).

Urban planning in the Netherlands has traditionally been far better than over here and the result is very short average trip lengths. The modal share of public transport is very low, nothing compared to London, and walking likewise (although it has to be pointed out that the piecharts only show modal share by primary mode used in each trip, while of course each trip by public transport is accessed by other modes, e.g. cycling or walking to the station).