You don't need to follow a buddy down the highway anymore now that GPS shows vehicle speed.
We tried doing that once and it was not as accurate. I even tried it on a boat while testing propellers and it was not even close. I guess for everyday purposes GPS will be OK, but here in Shitty City we have speed cameras on almost every block that use radar. Being off by even one or two MPH can get you a ticket. This gets problematic when you change tire size and it throws off your speedo by a small amount. The speed cameras are 'supposed' to give you a 5mph grace, but we have gotten tickets for being 2mph over. I blame speedo error.
