I don't think you can make a blanket statement like that. Where are your facts to back up that claim?
We have alot of our speedometers tested every three weeks and most are dead on accurate with a few understated by 2 mph.
I like how you always ask for facts but never back your options with facts....
Below is article that talks about today's Speedometers, which are electronic, and with the miracle of software and engineering the Speed the Dash Speedometer show can be programmed to show any speed they want it to show. Most new car today actually maintain two speeds the true speed which is used to calculated miles travel since car warranties are base on this and if this is not accurate everyone would sue the manufacturers for fraud. Then there is the the adjusted speed which the Dash Displaces to the driver. The problem is this, most car have analog Speedometers so it hard to know it it is reading 60 or may 61 or maybe 59. If you have digital gauge then you should know exactly what it is displaying.
Oh this article is from 2002. and I can tell most all car today read lower and your own statement backs that, since most likely NY is using Crownvic a Ford product.
Speedometer Scandal! - Feature - Auto Reviews - Car and Driver
http://www.accidentreconstruction.com/newsletter/aug08/ECM_SPEED_DATA.pdf
http://www.v3instruments.com/industrial_instruction_sheets/programmable_speedo_odo.pdf
http://www.pvdwiki.com/index.php?title=Speedometer_Calibration
Lastly, I have an engineer who sits right across from me who use to work for Ford and was responsible for the car computer systems and backed this up that what is displaced is not the actually the real car speed, usually is it slower unless the driver changes tires sizes or does something else to mess with the ratio.
Civilian signal GPS is not accurate enough to trust in getting a dead-accurate speed reading.
The quality of the receiver is not an issue - it's the satellite signal.
Again before you make an uninformed statement they are accurate enough and they are more accurate than you think especially if your receiver contains the SiRFstar III chip set which most receiver built in the last 4 year do. The satellite signal clocks are the most accurate in the world, they get their central time from the atomic clocks based in Colorado.
Here go educate yourself, all the technically specs are there
Welcome to SiRF Technology
If you not capable of understand that, do a simple test. find a section of highway which has exact 1 mile marked off, set your cruise control to 60 by your Speedometer and see what a good GSP with the SiRFStar III receiver in it and notice its speed then as you go past the first mark start a stop watch then when you cross the second mark see how close to 60sec you are, remember reaction time plays in.
Do this again setting your speed to 60 using the GPS and see what happens, more times than not the GPS will be closer.
Just so you understand, the reason civilian GPS are not "accurate" it has nothing to do with speed, it is absolute position which is not accurate. They will not pin point you, and it is not for technically reason but by design, they put in an inherent error into the system. however, the error is the same over time and distance so when calculating speed it is the change in distance over change in time and that is right on.
