Around here, they take the cruiser to a section of highway that has been measured as one mile. They get the cruiser up to the desired speed, hold it there for the mile, and time how long it takes to cover that distance, then calculate the actual speed.

IF you look at the bottom of a Crown Vic Police Interceptor speedo it says 'certified calibration' It is done at the factory and then I think its up to the department recertify it if needed.

Jay

mine used to be about 3-5off...

until i ripped apart my dash, and had to put the needles... 15 miles of country driving and i finally got em perfect...i think my boost gauge is still a lil low though

i recently had a similar problem....
it was caused by my bro.... he hit the english/metric button.