Don't know if Carman has been ported to Maemo5 yet, but I use it with a bluetooth-based OBD-II reader...
minigpsd which is in extras-devel also will tap into a bluetooth OBD-II reader -- it records the OBDII info along with the gps track so that you can look at it later. However, someone could probably build an interface to use the info logged by minigpsd.