Speed results are the same with all 4 GPS units I have used. I have a cheap off brand Car GPS from meijer, wife has a TOMTOM, I have had gps programs on 2 different cell phones and they all displayed the same exact MPH.
I haven't used a Garmin but I love my wife's TOMTOM. We paid extra so it verbally says the name of the street to turn on and it also tells you which lane to get in. Useful for traveling on the highway in a state you never been in before and/or any road that has many lanes and exit only lanes.
As long as it has signal my wife can drive anywhere. No more calling me lol !