I compared readings from car odometer and from GPS (Garmin Nuvi 350)

Car instruments usually display a higher than actual speed and a higher than actual distance, and you get the impression that your MPG is better that it is.

I assumed the same was true for my new freelander, but what a surprise!
100 km according to car odometer are 100,95 km according to GPS.

This implies that fuel economy is even slighly better that we tought

I've a similar experience with accuracy between GPS camera detector and speedo (so I've therefore assumed similar accuracy for the odomoeter). I reckon my speedo is 97% to 98% accurate to the GPS: speedo reads around 79 mph when GPS 77 mph. Also seems more accurate at higher speeds. DT