I can play that game too.
A certain stretch on my commute. Hit the 50 mph sign at exactly 50 mph, and coast until the spot where the pavement changes. Record the final speed. This was last spring, but I stopped bothering to record this data when it showed to be so very linear.
It would be even closer, but a couple of those low points showed me that my tires were a few psi low. Once I fixed that, the data got back in line.