What I have always used, here in the States, is the mile markers on the
interstate highways, look a my watch as I go past one, try to hold an even
60 MPH, look at my watch at the next one, should be one minute later, if
less, I am going more than 60, more than a minute, less that 60. At that
speed every second is about one mile per hour difference.
Cheers Dean Illinois