I too use this method to check my speedometer for accuracy.DiggerDerrik wrote:I have stock sized tires with roughly 10k miles on mine. I pass one of those speed limit signs with the digital readout of your speed and when my Speedo says 40 the sign reads 37-38. When I pass the same sign in my pickup going 40 the sign says 40-39. Granted the radar on that sign probably isn’t pinpoint but it’s consistent with both vehicles every time I pass it.
On a side note, if you find one that's set to a speed like 15 or 25, if you race through it at, say 60mph, it may blink in red vs yellow.
He was a design engineer at Ford for front steering and suspension back during that era. The Mercury Lincoln used in the movie was almost identical in so many features to our Mercury Monarch--so many memories. 