For the drive to Montréal, I borrowed a Tom-Tom GPS device — for fun, really; the way is straightforward. It’s the first time I’ve used one in my car, and I have to say that despite the advantage of portability, the built-in ones that I’ve seen in friends’ cars are far better, what with their larger screens and consequent improvement in user interface.
One thing the Tom-Tom does is show your driving speed. Because it’s measured from the GPS signals, I presume it’s accurate. And I found something interesting: my car’s speedometer reads about 5% high, at least at highway speeds. I had to go 68 or 69 MPH on my speedometer for the GPS to show 65 MPH (the speed limit on the Northway).[1]