You know what, I've set my GPS on the dashboard of my truck to check its accuracy, and it did pretty good. Considering that I was going up to 75 mph in the truck, and considering the max speed recorded by Manny and Julianne of well over 28mph, if you consider that their gps was a max of 6mph off, putting their speed at +22, I should have observed in excess of 12 mph differences in GPS speed vs speedo. Nope.
Also, I have oversized tires on the truck uncompensated by the odo, GPS shows about 6% faster than the odo and is consistent. Confirmed by police radar.
I think the inaccuracy of GPS may be overstated. Probably decimals in most cases. OMHO