Hi, my new-to-me 2003 K12 1.0 speedometer seem to under read, compared to the revs and GPS. (I read the car is geared 18.6 mph per 1k rpm.) At 2k rpm the clock says 33mph, should say 37 or so. At 2.5k rpm, speedo 40, should be 46. My GPS can be hard to lock on, but when it does it seems to say...