OpenXC had a really easy to way to get odometer readings, otherwise according to [1] "For a Ford vehicle, the odometer can be read via a mode $22 PID of $6180 for km clusters, and $6182 for mph clusters.". [2] Also talks a bit about reading Odometer. Shame there isn't a standard for reading it.

[1] http://www.mp3car.com/engine-management-obd-ii-engine-diagno...

[2] http://www.canbushack.com/blog/index.php?title=oh-no-odomete...