That is interesting. What gps did you use? I’m a sad as you as I’ve done similar things with all my cars for as long as gps has been available. (20 years?). In every case the car speedo has been higher than the gps readings. For example my Audi Q5 indicates 30 at gps 28mph.
It's the one from Demon Tweeks. All its done really is to confirm how accurate the car speedos are, except the Golf.
